Cymothoa exigua parassita Lithognathus mormyrus

Parasitism is a

close relationship An intimate relationship is an interpersonal relationship that involves emotional or physical closeness between people and may include sexual intimacy and feelings of Romance (love), romance or love. Intimate relationships are Interdependence ...

between

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County * Host Island, in the Wilhelm Archipelago, Antarctica People * ...

, causing it some harm, and is adapted structurally to this way of life. The entomologist

E. O. Wilson Edward Osborne Wilson (June 10, 1929 – December 26, 2021) was an American biologist, naturalist, ecologist, and entomologist known for developing the field of sociobiology. Born in Alabama, Wilson found an early interest in nature and frequ ...

characterised parasites' way of feeding as "predators that eat prey in units of less than one". Parasites include single-celled

protozoa Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically ...

ns such as the agents of

malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects vertebrates and ''Anopheles'' mosquitoes. Human malaria causes Signs and symptoms, symptoms that typically include fever, Fatigue (medical), fatigue, vomitin ...

sleeping sickness African trypanosomiasis is an insect-borne parasitic infection of humans and other animals. Human African trypanosomiasis (HAT), also known as African sleeping sickness or simply sleeping sickness, is caused by the species '' Trypanosoma b ...

, and

amoebic dysentery Amoebiasis, or amoebic dysentery, is an infection of the intestines caused by a parasitic amoeba '' Entamoeba histolytica''. Amoebiasis can be present with no, mild, or severe symptoms. Symptoms may include lethargy, loss of weight, coloni ...

; animals such as

hookworm Hookworms are Gastrointestinal tract, intestinal, Hematophagy, blood-feeding, parasitic Nematode, roundworms that cause types of infection known as helminthiases. Hookworm infection is found in many parts of the world, and is common in areas with ...

lice Louse (: lice) is the common name for any member of the infraorder Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera was previously recognized as an order, until a 2021 genetic study determined th ...

mosquito Mosquitoes, the Culicidae, are a Family (biology), family of small Diptera, flies consisting of 3,600 species. The word ''mosquito'' (formed by ''Musca (fly), mosca'' and diminutive ''-ito'') is Spanish and Portuguese for ''little fly''. Mos ...

es, and

vampire bat Vampire bats, members of the subfamily Desmodontinae, are Phyllostomidae, leaf-nosed bats currently found in Central and South America. Their food source is the blood of other animals, a dietary trait called hematophagy. Three extant bat species ...

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

such as

honey fungus ''Armillaria'' is a genus of fungi that includes the '' A. mellea'' species ('honey fungus') that live on trees and woody shrubs. It includes about 10 species formerly categorized summarily as ''A. mellea''. ''Armillaria'' sp. are long-live ...

and the agents of

ringworm Dermatophytosis, also known as tinea and ringworm, is a mycosis, fungal infection of the skin (a dermatomycosis), that may affect skin, hair, and nails. Typically it results in a red, itchy, scaly, circular rash. Hair loss may occur in the a ...

; and plants such as

mistletoe Mistletoe is the common name for obligate parasite, obligate parasitic plant, hemiparasitic plants in the Order (biology), order Santalales. They are attached to their host tree or shrub by a structure called the haustorium, through which they ...

dodder ''Cuscuta'' (), commonly known as dodder or amarbel, is a genus of over 201 species of yellow, orange, or red (rarely green) parasitic plants. Formerly treated as the only genus in the family Cuscutaceae, it now is accepted as belonging in the ...

, and the broomrapes. There are six major parasitic

strategies Strategy (from Greek στρατηγία ''stratēgia'', "troop leadership; office of general, command, generalship") is a general plan to achieve one or more long-term or overall goals under conditions of uncertainty. In the sense of the "art o ...

of exploitation of animal hosts, namely

parasitic castration Parasitic castration is the strategy, by a parasite, of blocking reproduction by its host, completely or in part, to its own benefit. This is one of six major strategies within parasitism. Evolutionary strategy The parasitic castration strateg ...

, directly transmitted parasitism (by contact), trophicallytransmitted parasitism (by being eaten), vector-transmitted parasitism,

parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...

ism, and micropredation. One major axis of classification concerns invasiveness: an endoparasite lives inside the host's body; an ectoparasite lives outside, on the host's surface. Like predation, parasitism is a type of consumer–resource interaction, but unlike

predators Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...

, parasites, with the exception of parasitoids, are much smaller than their hosts, do not kill them, and often live in or on their hosts for an extended period. Parasites of animals are highly specialised, each parasite species living on one given animal species, and

reproduce Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. There are two forms of reproduction: asexual and sexual. In asexual reprod ...

at a faster rate than their hosts. Classic examples include interactions between

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

hosts and

tapeworms Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, commonly known as tapeworms. Their bodies con ...

, flukes, and those between the malaria-causing ''

Plasmodium ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a Hematophagy, blood-feeding insect host (biology), host which then inj ...

'' species, and

flea Flea, the common name for the order (biology), order Siphonaptera, includes 2,500 species of small flightless insects that live as external parasites of mammals and birds. Fleas live by hematophagy, ingesting the blood of their hosts. Adult f ...

s. Parasites reduce host fitness by general or specialised

pathology Pathology is the study of disease. The word ''pathology'' also refers to the study of disease in general, incorporating a wide range of biology research fields and medical practices. However, when used in the context of modern medical treatme ...

, that ranges from

to modification of host behaviour. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular by feeding on them and by using intermediate (secondary) hosts to assist in their

transmission Transmission or transmit may refer to: Science and technology * Power transmission ** Electric power transmission ** Transmission (mechanical device), technology that allows controlled application of power *** Automatic transmission *** Manual tra ...

from one definitive (primary) host to another. Although parasitism is often unambiguous, it is part of a spectrum of interactions between

, grading via parasitoidism into predation, through evolution into mutualism, and in some fungi, shading into being

saprophytic Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ...

. Human knowledge of parasites such as

roundworm The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (hel ...

s and

tapeworm Eucestoda, commonly referred to as tapeworms, is the larger of the two subclasses of flatworms in the class Cestoda (the other subclass being Cestodaria). Larvae have six posterior hooks on the scolex (head), in contrast to the ten-hooked Ce ...

s dates back to

ancient Egypt Ancient Egypt () was a cradle of civilization concentrated along the lower reaches of the Nile River in Northeast Africa. It emerged from prehistoric Egypt around 3150BC (according to conventional Egyptian chronology), when Upper and Lower E ...

Greece Greece, officially the Hellenic Republic, is a country in Southeast Europe. Located on the southern tip of the Balkan peninsula, it shares land borders with Albania to the northwest, North Macedonia and Bulgaria to the north, and Turkey to th ...

, and

Rome Rome (Italian language, Italian and , ) is the capital city and most populated (municipality) of Italy. It is also the administrative centre of the Lazio Regions of Italy, region and of the Metropolitan City of Rome. A special named with 2, ...

. In

early modern The early modern period is a Periodization, historical period that is defined either as part of or as immediately preceding the modern period, with divisions based primarily on the history of Europe and the broader concept of modernity. There i ...

times,

Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch microbiologist and microscopist in the Golden Age of Dutch art, science and technology. A largely self-taught man in science, he is commonly known as " ...

observed ''

Giardia lamblia ''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellated Parasitism, parasitic protozoan microorganism of the genus ''Giardia'' that colonizes the small intestine, causing a diarrheal condition kn ...

'' with his microscope in 1681, while

Francesco Redi Francesco Redi (18 February 1626 – 1 March 1697) was an Italians, Italian physician, naturalist, biologist, and poet. He is referred to as the "founder of experimental biology", and as the "father of modern parasitology". He was the first perso ...

described internal and external parasites including

sheep liver fluke ''Fasciola hepatica'', also known as the common liver fluke or sheep liver fluke, is a parasitic trematode ( fluke or flatworm, a type of helminth) of the class Trematoda, phylum Platyhelminthes. It infects the livers of various mammals, inc ...

and

tick Ticks are parasitic arachnids of the order Ixodida. They are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, and species, but can become larger when engorged. Ticks a ...

s. Modern

parasitology Parasitology is the study of parasites, their host (biology), hosts, and the relationship between them. As a List of biology disciplines, biological discipline, the scope of parasitology is not determined by the organism or environment in questio ...

developed in the 19th century. In human culture, parasitism has negative connotations. These were exploited to

satirical Satire is a genre of the visual arts, visual, literature, literary, and performing arts, usually in the form of fiction and less frequently Nonfiction, non-fiction, in which vices, follies, abuses, and shortcomings are held up to ridicule, ...

effect in

Jonathan Swift Jonathan Swift (30 November 1667 – 19 October 1745) was an Anglo-Irish writer, essayist, satirist, and Anglican cleric. In 1713, he became the Dean (Christianity), dean of St Patrick's Cathedral, Dublin, and was given the sobriquet "Dean Swi ...

's 1733 poem "On Poetry: A Rhapsody", comparing poets to

hyperparasitical A hyperparasite, also known as a metaparasite, is a parasite whose host, often an insect, is also a parasite, often specifically a parasitoid. Hyperparasites are found mainly among the wasp-waisted Apocrita within the Hymenoptera, and in two ot ...

"vermin". In fiction,

Bram Stoker Abraham Stoker (8 November 1847 – 20 April 1912), better known by his pen name Bram Stoker, was an Irish novelist who wrote the 1897 Gothic horror novel ''Dracula''. The book is widely considered a milestone in Vampire fiction, and one of t ...

's 1897

Gothic horror Gothic fiction, sometimes referred to as Gothic horror (primarily in the 20th century), is a literary aesthetic of fear and haunting. The name of the genre is derived from the Renaissance era use of the word "gothic", as a pejorative to mean m ...

novel ''

Dracula ''Dracula'' is an 1897 Gothic fiction, Gothic horror fiction, horror novel by Irish author Bram Stoker. The narrative is Epistolary novel, related through letters, diary entries, and newspaper articles. It has no single protagonist and opens ...

'' and its many later adaptations featured a blood-drinking parasite.

Ridley Scott Sir Ridley Scott (born 30 November 1937) is an English film director and producer. He directs films in the Science fiction film, science fiction, Crime film, crime, and historical drama, historical epic genres, with an atmospheric and highly co ...

's 1979 film '' Alien'' was one of many works of

science fiction Science fiction (often shortened to sci-fi or abbreviated SF) is a genre of speculative fiction that deals with imaginative and futuristic concepts. These concepts may include information technology and robotics, biological manipulations, space ...

to feature a parasitic alien species.

Etymology

First used in English in 1539, the word ''parasite'' comes from the

Medieval French Old French (, , ; ) was the language spoken in most of the northern half of France approximately between the late 8th Latinised form , . The related term ''parasitism'' appears in English from 1611.

Evolutionary strategies

Basic concepts

Taenia solium tapeworm scolex with its four suckers and two rows of hooks 5262 lores

Parasitism is a kind of symbiosis, a close and persistent long-term biological interaction between a parasite and its host. Unlike saprotrophs, parasites feed on living hosts, though some parasitic fungi, for instance, may continue to feed on hosts they have killed. Unlike

commensalism Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fr ...

and mutualism, the parasitic relationship harms the host, either feeding on it or, as in the case of intestinal parasites, consuming some of its food. Because parasites interact with other species, they can readily act as vectors of pathogens, causing

disease A disease is a particular abnormal condition that adversely affects the structure or function (biology), function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical condi ...

Predation Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

is by definition not a symbiosis, as the interaction is brief, but the entomologist

has characterised parasites as "predators that eat prey in units of less than one". Within that scope are many possible strategies.

Taxonomist In biology, taxonomy () is the science, scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxon, taxa (si ...

s classify parasites in a variety of overlapping schemes, based on their interactions with their hosts and on their life cycles, which can be complex. An

obligate parasite An obligate parasite or holoparasite is a parasitic organism that cannot complete its life-cycle without exploiting a suitable host. If an obligate parasite cannot obtain a host it will fail to reproduce. This is opposed to a facultative parasite, ...

depends completely on the host to complete its life cycle, while a

facultative parasite A facultative parasite is an organism that may resort to parasitic activity, but does not absolutely rely on any host for completion of its life cycle. Examples of facultative parasitism occur among many species of fungi, such as family members ...

does not. Parasite life cycles involving only one host are called "direct"; those with a definitive host (where the parasite reproduces sexually) and at least one intermediate host are called "indirect". An endoparasite lives inside the host's body; an ectoparasite lives outside, on the host's surface. Mesoparasites—like some

copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...

s, for example—enter an opening in the host's body and remain partly embedded there. Some parasites can be generalists, feeding on a wide range of hosts, but many parasites, and the majority of protozoans and

helminth Parasitic worms, also known as helminths, are a polyphyletic group of large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other par ...

s that parasitise animals, are specialists and extremely host-specific. An early basic, functional division of parasites distinguished microparasites and macroparasites. These each had a

mathematical model A mathematical model is an abstract and concrete, abstract description of a concrete system using mathematics, mathematical concepts and language of mathematics, language. The process of developing a mathematical model is termed ''mathematical m ...

assigned in order to analyse the population movements of the host–parasite groupings. The microorganisms and viruses that can reproduce and complete their life cycle within the host are known as microparasites. Macroparasites are the multicellular organisms that reproduce and complete their life cycle outside of the host or on the host's body. Much of the thinking on types of parasitism has focused on terrestrial animal parasites of animals, such as helminths. Those in other environments and with other hosts often have analogous strategies. For example, the

snubnosed eel The snubnosed eel, ''Simenchelys parasitica'', also known as the pug-nosed eel, slime eel, or snub-nose parasitic eel, is a species of deep-sea eel and the only member of its genus. Some authors classify it as the sole member of the subfamily Si ...

is probably a facultative endoparasite (i.e., it is semiparasitic) that opportunistically burrows into and eats sick and dying fish. Plant-eating insects such as

scale insect Scale insects are small insects of the Order (biology), order Hemiptera, suborder Sternorrhyncha. Of dramatically variable appearance and extreme sexual dimorphism, they comprise the infraorder Coccomorpha which is considered a more convenient g ...

aphid Aphids are small sap-sucking insects in the Taxonomic rank, family Aphididae. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white Eriosomatinae, woolly ...

s, and

caterpillar Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder ...

s closely resemble ectoparasites, attacking much larger plants; they serve as vectors of bacteria, fungi and viruses which cause

plant diseases Plant diseases are diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like orga ...

. As female scale insects cannot move, they are obligate parasites, permanently attached to their hosts. The sensory inputs that a parasite employs to identify and approach a potential host are known as "host cues". Such cues can include, for example, vibration, exhaled

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

, skin odours, visual and heat signatures, and moisture. Parasitic plants can use, for example, light, host physiochemistry, and volatiles to recognize potential hosts.

Major strategies

There are six major parasitic

, namely

; directly transmitted parasitism; trophically-transmitted parasitism;

vector Vector most often refers to: * Euclidean vector, a quantity with a magnitude and a direction * Disease vector, an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematics a ...

-transmitted parasitism;

ism; and micropredation. These apply to parasites whose hosts are plants as well as animals. These strategies represent adaptive peaks; intermediate strategies are possible, but organisms in many different groups have consistently converged on these six, which are evolutionarily stable. A perspective on the evolutionary options can be gained by considering four key questions: the effect on the fitness of a parasite's hosts; the number of hosts they have per life stage; whether the host is prevented from reproducing; and whether the effect depends on intensity (number of parasites per host). From this analysis, the major evolutionary strategies of parasitism emerge, alongside predation.

Parasitic castrators

Parasitic castrators partly or completely destroy their host's ability to reproduce, diverting the energy that would have gone into reproduction into host and parasite growth, sometimes causing gigantism in the host. The host's other systems remain intact, allowing it to survive and to sustain the parasite. Parasitic crustaceans such as those in the specialised

barnacle Barnacles are arthropods of the subclass (taxonomy), subclass Cirripedia in the subphylum Crustacean, Crustacea. They are related to crabs and lobsters, with similar Nauplius (larva), nauplius larvae. Barnacles are exclusively marine invertebra ...

genus ''

Sacculina ''Sacculina'' is a genus of barnacles that is a parasitic castrator of crabs. They belong to a group called Rhizocephala. The adults bear no resemblance to the barnacles that cover ships, whales, and piers; they are recognised as barnacles beca ...

'' specifically cause damage to the gonads of their many species of host

crab Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek language, Greek), which typically have a very short projecting tail-like abdomen#Arthropoda, abdomen, usually hidden entirely under the Thorax (arthropo ...

s. In the case of ''Sacculina'', the testes of over two-thirds of their crab hosts degenerate sufficiently for these male crabs to develop female

secondary sex characteristic A secondary sex characteristic is a physical characteristic of an organism that is related to or derived from its sex, but not directly part of its reproductive system. In humans, these characteristics typically start to appear during pubert ...

s such as broader abdomens, smaller

claws A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or tars ...

and egg-grasping appendages. Various species of helminth castrate their hosts (such as insects and snails). This may happen directly, whether mechanically by feeding on their gonads, or by secreting a chemical that destroys reproductive cells; or indirectly, whether by secreting a hormone or by diverting nutrients. For example, the

trematode Trematoda is a Class (biology), class of flatworms known as trematodes, and commonly as flukes. They are obligate parasite, obligate Endoparasites, internal parasites with a complex biological life cycle, life cycle requiring at least two Host ( ...

'' Zoogonus lasius'', whose sporocysts lack mouths, castrates the intertidal marine snail '' Tritia obsoleta'' chemically, developing in its gonad and killing its reproductive cells.

Directly transmitted

Directly transmitted parasites, not requiring a vector to reach their hosts, include such parasites of terrestrial vertebrates as lice and mites; marine parasites such as

s and cyamid amphipods;

monogenea Monogeneans, members of the class Monogenea, are a group of ectoparasitic flatworms commonly found on the skin, gills, or fins of fish. They have a direct lifecycle and do not require an intermediate host. Adults are hermaphrodites, meaning they ...

ns; and many species of nematodes, fungi, protozoans, bacteria, and viruses. Whether endoparasites or ectoparasites, each has a single host-species. Within that species, most individuals are free or almost free of parasites, while a minority carry a large number of parasites; this is known as an

aggregated distribution An aggregated distribution, commonly found among predators, parasites, and plants is a highly uneven (skewed) statistical distribution pattern in which they collect or aggregate in regions. These regions may be widely separated, particularly within ...

Trophically transmitted

Trophically-transmitted parasites are transmitted by being eaten by a host. They include trematodes (all except schistosomes),

cestodes Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, commonly known as tapeworms. Their bodies con ...

acanthocephala Acanthocephala ( Greek , ' 'thorn' + , ' 'head') is a group of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an eversible proboscis, armed with spines, which it uses t ...

ns, pentastomids, many

roundworms The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (hel ...

, and many protozoa such as ''

Toxoplasma ''Toxoplasma gondii'' () is a species of parasitic alveolate that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-blooded animals, but members of the cat family (felidae) are the only known d ...

''. They have complex life cycles involving hosts of two or more species. In their juvenile stages they infect and often

encyst A microbial cyst is a resting or dormant stage of a microorganism, that can be thought of as a state of suspended animation in which the metabolic processes of the cell are slowed and the cell ceases all activities like feeding and locomotion. ...

in the intermediate host. When the intermediate-host animal is eaten by a predator, the definitive host, the parasite survives the digestion process and matures into an adult; some live as

intestinal parasite An intestinal parasite infection is a condition in which a parasite infects the gastro-intestinal tract of humans and other animals. Such parasites can live anywhere in the body, but most prefer the intestinal wall. Routes of exposure and inf ...

s. Many trophically transmitted parasites modify the behaviour of their intermediate hosts, increasing their chances of being eaten by a predator. As with directly transmitted parasites, the distribution of trophically transmitted parasites among host individuals is aggregated.

Coinfection Coinfection is the simultaneous infection of a host by multiple pathogen species. In virology, coinfection includes simultaneous infection of a single cell by two or more virus particles. An example is the coinfection of liver cells with hepatit ...

by multiple parasites is common. Autoinfection, where (by exception) the whole of the parasite's life cycle takes place in a single primary host, can sometimes occur in helminths such as ''

Strongyloides stercoralis ''Strongyloides stercoralis'' is a human pathogenic parasitic roundworm causing the disease strongyloidiasis. Its common name in the US is threadworm. In the UK and Australia, however, the term ''threadworm'' can also refer to nematodes of t ...

''.

Vector-transmitted

Vector-transmitted parasites rely on a third party, an intermediate host, where the parasite does not reproduce sexually, to carry them from one definitive host to another. These parasites are microorganisms, namely

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

, or

virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...

es, often intracellular

pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...

s (disease-causers). Their vectors are mostly

hematophagic Hematophagy (sometimes spelled haematophagy or hematophagia) is the practice by certain animals of feeding on blood (from the Greek words αἷμα ' "blood" and φαγεῖν ' "to eat"). Since blood is a fluid tissue rich in nutritious pr ...

arthropods such as fleas, lice, ticks, and mosquitoes. For example, the deer tick ''

Ixodes scapularis ''Ixodes scapularis'' is a hard-bodied tick found in much of the eastern half of North America. It is commonly known as the deer tick, owing to its habit of parasitizing the white-tailed deer. It is also sometimes known as the black-legged tick ( ...

'' acts as a vector for diseases including

Lyme disease Lyme disease, also known as Lyme borreliosis, is a tick-borne disease caused by species of ''Borrelia'' bacteria, Disease vector, transmitted by blood-feeding ticks in the genus ''Ixodes''. It is the most common disease spread by ticks in th ...

babesiosis Babesiosis or piroplasmosis is a malaria-like parasitic disease caused by infection with a eukaryotic parasite in the order Piroplasmida, typically a ''Babesia'' or '' Theileria'', in the phylum Apicomplexa. Human babesiosis transmission via ...

, and

anaplasmosis Anaplasmosis is a tick-borne disease affecting ruminants, dogs, and horses, and is caused by ''Anaplasma'' bacteria. Anaplasmosis is an infectious but not contagious disease. Anaplasmosis can be transmitted through mechanical and biological vector ...

. Protozoan endoparasites, such as the

l parasites in the genus ''

'' and sleeping-sickness parasites in the genus ''

Trypanosoma ''Trypanosoma'' is a genus of kinetoplastids (class Trypanosomatidae), a monophyletic group of unicellular parasitic flagellate protozoa. Trypanosoma is part of the phylum Euglenozoa. The name is derived from the Ancient Greek ''trypano-'' (b ...

'', have infective stages in the host's blood which are transported to new hosts by biting insects.

Parasitoids

Parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...

s are insects which sooner or later kill their hosts, placing their relationship close to predation. Most parasitoids are

parasitoid wasp Parasitoid wasps are a large group of hymenopteran Superfamily (zoology), superfamilies, with all but the wood wasps (Orussoidea) being in the wasp-waisted Apocrita. As parasitoids, they lay their eggs on or in the bodies of other arthropods, ...

s or other

hymenoptera Hymenoptera is a large order of insects, comprising the sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are parasitic. Females typi ...

ns; others include

dipterans Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...

such as phorid flies. They can be divided into two groups, idiobionts and koinobionts, differing in their treatment of their hosts. Idiobiont parasitoids sting their often-large prey on capture, either killing them outright or paralysing them immediately. The immobilised prey is then carried to a nest, sometimes alongside other prey if it is not large enough to support a parasitoid throughout its development. An egg is laid on top of the prey and the nest is then sealed. The parasitoid develops rapidly through its larval and pupal stages, feeding on the provisions left for it. Koinobiont parasitoids, which include

flies Flies are insects of the Order (biology), order Diptera, the name being derived from the Ancient Greek, Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwin ...

as well as wasps, lay their eggs inside young hosts, usually larvae. These are allowed to go on growing, so the host and parasitoid develop together for an extended period, ending when the parasitoids emerge as adults, leaving the prey dead, eaten from inside. Some koinobionts regulate their host's development, for example preventing it from

pupa A pupa (; : pupae) is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages th ...

ting or making it

moult In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is a process by which an animal casts off parts of its body to serve some beneficial purpose, either at ...

whenever the parasitoid is ready to moult. They may do this by producing hormones that mimic the host's moulting hormones (

ecdysteroid Ecdysteroids are arthropod steroid hormones that are mainly responsible for molting (ecdysis), development and, to a lesser extent, reproduction; examples of ecdysteroids include ecdysone, 20-hydroxyecdysone (ecdysterone), turkesterone and 2- ...

s), or by regulating the host's endocrine system. File:Live Tetragnatha montana (RMNH.ARA.14127) parasitized by Acrodactyla quadrisculpta larva (RMNH.INS.593867) - BDJ.1.e992.jpg , Idiobiont

s immediately paralyse their hosts for their larvae (

Pimplinae Pimplinae are a worldwide subfamily of the parasitic wasp family Ichneumonidae. Pimplinae are parasitoids of Holometabola, often the pupae of Lepidoptera Lepidoptera ( ) or lepidopterans is an order (biology), order of winged insects whi ...

, pictured) to eat. File:CSIRO ScienceImage 2357 Spotted alfalfa aphid being attacked by parasitic wasp.jpg, Koinobiont parasitoid wasps like this braconid lay their eggs via an

ovipositor The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typica ...

inside their hosts, which continue to grow and moult. File:Female Apocephalus borealis ovipositing into the abdomen of a worker honey bee.png,

Phorid fly The Phoridae are a family (biology), family of small, hump-backed Fly, flies resembling Drosophila melanogaster, fruit flies. Phorid flies can often be identified by their escape habit of running rapidly across a surface rather than taking flight ...

(centre left) is laying eggs in the abdomen of a worker honey-bee, altering its behaviour.

Micropredators

A micropredator attacks more than one host, reducing each host's fitness by at least a small amount, and is only in contact with any one host intermittently. This behavior makes micropredators suitable as vectors, as they can pass smaller parasites from one host to another. Most micropredators are

, feeding on blood. They include annelids such as

leech Leeches are segmented parasitism, parasitic or Predation, predatory worms that comprise the Class (biology), subclass Hirudinea within the phylum Annelida. They are closely related to the Oligochaeta, oligochaetes, which include the earthwor ...

es, crustaceans such as

branchiura The family Argulidae, whose members are commonly known as carp lice or fish lice, are parasitic crustaceans in the class Ichthyostraca. It is the only family in the monotypic subclass Branchiura and the order Arguloida, although a second family ...

ns and gnathiid isopods, various

diptera Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advance ...

ns such as mosquitoes and

tsetse flies Tsetse ( , or ) (sometimes spelled tzetze; also known as tik-tik flies) are large, biting flies that inhabit much of tropical Africa. Tsetse flies include all the species in the genus ''Glossina'', which are placed in their own family, Glos ...

, other arthropods such as fleas and ticks, vertebrates such as

lamprey Lampreys (sometimes inaccurately called lamprey eels) are a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterize ...

s, and mammals such as

Transmission strategies

Parasites use a variety of methods to infect animal hosts, including physical contact, the

fecal–oral route The fecal–oral route (also called the oral–fecal route or orofecal route) describes a particular route of transmission of a disease wherein pathogens in fecal particles pass from one person to the mouth of another person. Main causes of fec ...

, free-living infectious stages, and vectors, suiting their differing hosts, life cycles, and ecological contexts. Examples to illustrate some of the many possible combinations are given in the table.

Variations

Among the many variations on parasitic strategies are hyperparasitism, social parasitism, brood parasitism, kleptoparasitism, sexual parasitism, and adelphoparasitism.

Hyperparasitism

Hyperparasite A hyperparasite, also known as a metaparasite, is a parasite whose host, often an insect, is also a parasite, often specifically a parasitoid. Hyperparasites are found mainly among the wasp-waisted Apocrita within the Hymenoptera, and in two ot ...

s feed on another parasite, as exemplified by protozoa living in helminth parasites, or facultative or obligate parasitoids whose hosts are either conventional parasites or parasitoids. Levels of parasitism beyond secondary also occur, especially among facultative parasitoids. In

oak gall An oak apple or oak gall is a large, round, vaguely apple-like gall commonly found on many species of oak. Oak apples range in size from in diameter and are caused by chemicals injected by the larva of certain kinds of gall wasp in the family ...

systems, there can be up to four levels of parasitism. Hyperparasites can control their hosts' populations, and are used for this purpose

in agriculture IN, In or in may refer to: Dans * India (country code IN) * Indiana, United States (postal code IN) * Ingolstadt, Germany (license plate code IN) * In, Russia, a town in the Jewish Autonomous Oblast Businesses and organizations * Independen ...

and to some extent in

medicine Medicine is the science and Praxis (process), practice of caring for patients, managing the Medical diagnosis, diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment, Palliative care, palliation of their injury or disease, ...

. The controlling effects can be seen in the way that the CHV1 virus helps to control the damage that

chestnut blight The pathogenic fungus ''Cryphonectria parasitica'' (formerly ''Endothia parasitica'') is a member of the Ascomycota (sac fungi). This necrotrophic fungus is native to East Asia and South East Asia and was introduced into Europe and North America ...

, ''Cryphonectria parasitica'', does to

American chestnut The American chestnut (''Castanea dentata'') is a large, fast-growing deciduous tree of the Fagaceae, beech family native to eastern North America. As is true of all species in the genus ''Chestnut, Castanea'', the American chestnut produces Bur ...

trees, and in the way that

bacteriophage A bacteriophage (), also known informally as a phage (), is a virus that infects and replicates within bacteria. The term is derived . Bacteriophages are composed of proteins that Capsid, encapsulate a DNA or RNA genome, and may have structu ...

s can limit bacterial infections. It is likely, though little researched, that most pathogenic microparasites have hyperparasites which may prove widely useful in both agriculture and medicine.

Social parasitism

Social parasites take advantage of interspecific interactions between members of

eusocial Eusociality ( Greek 'good' and social) is the highest level of organization of sociality. It is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping generations wit ...

animals such as

ant Ants are Eusociality, eusocial insects of the Family (biology), family Formicidae and, along with the related wasps and bees, belong to the Taxonomy (biology), order Hymenoptera. Ants evolved from Vespoidea, vespoid wasp ancestors in the Cre ...

termite Termites are a group of detritivore, detritophagous Eusociality, eusocial cockroaches which consume a variety of Detritus, decaying plant material, generally in the form of wood, Plant litter, leaf litter, and Humus, soil humus. They are dist ...

s, and

bumblebee A bumblebee (or bumble bee, bumble-bee, or humble-bee) is any of over 250 species in the genus ''Bombus'', part of Apidae, one of the bee families. This genus is the only Extant taxon, extant group in the tribe Bombini, though a few extinct r ...

s. Examples include the large blue butterfly, ''

Phengaris arion The large blue (''Phengaris arion'') is a species of butterfly in the family Lycaenidae. The species was first defined in 1758 and first recorded in Britain in 1795. In 1979 the species became mostly extinct in Britain but has been successfully ...

'', its larvae employing

ant mimicry Ant mimicry or myrmecomorphy is mimicry of ants by other organisms; it has evolved over 70 times. Ants are abundant all over the world, and potential predators that rely on vision to identify their prey, such as birds and wasps, normally avoid t ...

to parasitise certain ants, '' Bombus bohemicus'', a bumblebee which invades the hives of other bees and takes over reproduction while their young are raised by host workers, and ''

Melipona scutellaris ''Melipona scutellaris'' is a eusocial stingless bee species of the order Hymenoptera and the genus ''Melipona''. It is considered to be the reared ''Melipona'' species with the largest distribution in the North and Northeast regions of Brazil, w ...

'', a eusocial bee whose virgin queens escape killer workers and invade another colony without a queen. An extreme example of interspecific social parasitism is found in the ant '' Tetramorium inquilinum'', an obligate parasite which lives exclusively on the backs of other ''Tetramorium'' ants. A mechanism for the evolution of social parasitism was first proposed by Carlo Emery in 1909. Now known as "

Emery's rule Emery's rule is the trend of social parasites to be parasites to species or genera they are closely related to. History In 1909, the Italian entomologist Carlo Emery noted that social parasites among insects (e.g., kleptoparasites) tend to be p ...

", it states that social parasites tend to be closely related to their hosts, often being in the same genus. Intraspecific social parasitism occurs in parasitic nursing, where some individual young take milk from unrelated females. In

wedge-capped capuchin The wedge-capped capuchin or Guianan weeper capuchin (''Cebus olivaceus'') is a capuchin monkey from South America. It is found in northern Brazil, Guyana and Venezuela. ''Cebus olivaceus'' is known to dwell in tall, primary forest and travel ove ...

s, higher ranking females sometimes take milk from low ranking females without any reciprocation.

Brood parasitism

brood parasitism Brood parasitism is a subclass of parasitism and phenomenon and behavioural pattern of animals that rely on others to raise their young. The strategy appears among birds, insects and fish. The brood parasite manipulates a host, either of the ...

, the hosts suffer increased parental investment and energy expenditure to feed parasitic young, which are commonly larger than host young. The growth rate of host nestlings is slowed, reducing the host's fitness. Brood parasites include birds in different families such as

cowbird Cowbirds are birds belonging to the genus ''Molothrus'' in the family Icteridae. They are of New World origin, but some species not native to North America are invasive there, and are obligate brood parasites, laying their eggs in the nests of o ...

s, whydahs,

cuckoo Cuckoos are birds in the Cuculidae ( ) family, the sole taxon in the order Cuculiformes ( ). The cuckoo family includes the common or European cuckoo, roadrunners, koels, malkohas, couas, coucals, and anis. The coucals and anis are somet ...

s, and

black-headed duck The black-headed duck (''Heteronetta atricapilla'') is a South American duck in subfamily Oxyurinae of family Anatidae.HBW and BirdLife International (2021) Handbook of the Birds of the World and BirdLife International digital checklist of th ...

s. These do not build nests of their own, but leave their eggs in nests of other

. In the family ''

Cuculidae Cuckoos are birds in the Cuculidae ( ) family, the sole taxon in the order Cuculiformes ( ). The cuckoo family includes the common or European cuckoo, roadrunners, koels, malkohas, couas, coucals, and anis. The coucals and anis are someti ...

'', over 40% of cuckoo species are obligate brood parasites, while others are either facultative brood parasites or provide parental care. The eggs of some brood parasites

mimic In evolutionary biology, mimicry is an evolved resemblance between an organism and another object, often an organism of another species. Mimicry may evolve between different species, or between individuals of the same species. In the simples ...

those of their hosts, while some cowbird eggs have tough shells, making them hard for the hosts to kill by piercing, both mechanisms implying selection by the hosts against parasitic eggs. The adult female European cuckoo further mimics a predator, the European sparrowhawk, giving her time to lay her eggs in the host's nest unobserved. Host species often combat parasitic egg mimicry through egg polymorphism, having two or more egg phenotypes within a single population of a species. Multiple phenotypes in host eggs decrease the probability of a parasitic species accurately "matching" their eggs to host eggs.

Kleptoparasitism

kleptoparasitism Kleptoparasitism (originally spelt clepto-parasitism, meaning "parasitism by theft") is a form of feeding in which one animal deliberately takes food from another. The strategy is evolutionarily stable when stealing is less costly than direct f ...

(from Greek κλέπτης (''kleptēs''), "thief"), parasites steal food gathered by the host. The parasitism is often on close relatives, whether within the same species or between species in the same genus or family. For instance, the many lineages of

cuckoo bee The term cuckoo bee is used for a variety of different bee lineages which have evolved the kleptoparasitic behaviour of laying their eggs in the nests of other bees, reminiscent of the behavior of cuckoo birds. The name is perhaps best applied t ...

s lay their eggs in the nest cells of other

bee Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfamil ...

s in the same family. Kleptoparasitism is uncommon generally but conspicuous in birds; some such as

skuas The skuas are a group of predatory seabirds with seven species forming the genus ''Stercorarius'', the only genus in the family Stercorariidae. The three smaller skuas, the Arctic skua, the long-tailed skua, and the pomarine skua, are called ...

are specialised in pirating food from other seabirds, relentlessly chasing them down until they disgorge their catch.

Sexual parasitism

A unique approach is seen in some species of

anglerfish The anglerfish are ray-finned fish in the order Lophiiformes (). Both the order's common name, common and scientific name comes from the characteristic mode of predation, in which a modified dorsal Fish fin#Ray-fins, fin ray acts as a Aggressiv ...

, such as ''

Ceratias holboelli ''Ceratias'' is a genus of marine ray-finned fish sometimes referred to as the doublewart- or giant seadevils, belonging to the family Ceratiidae, the warty sea devils. The fishes in this genus are found throughout the world's oceans. Taxonomy ...

'', where the males are reduced to tiny sexual parasites, wholly dependent on females of their own species for survival, permanently attached below the female's body, and unable to fend for themselves. The female nourishes the male and protects him from predators, while the male gives nothing back except the sperm that the female needs to produce the next generation.

Adelphoparasitism

Adelphoparasitism, (from Greek ἀδελφός (''adelphós''), brother), also known as sibling-parasitism, occurs where the host species is closely related to the parasite, often in the same family or genus. In the citrus blackfly parasitoid, '' Encarsia perplexa'', unmated females may lay

haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

eggs in the fully developed larvae of their own species, producing male offspring, while the marine worm '' Bonellia viridis'' has a similar reproductive strategy, although the larvae are planktonic.

Illustrations

Examples of the major variant strategies are illustrated. File:Pteromalid hyperparasitoid.jpg, A hyperparasitoid pteromalid wasp on the cocoons of its host, itself a parasitoid

braconid wasp The Braconidae are a family of parasitoid wasps. After the closely related Ichneumonidae, braconids make up the second-largest family in the order Hymenoptera, with about 17,000 recognized species and many thousands more undescribed. One analysis ...

File:Maculinea arion Large Blue Upperside SFrance 2009-07-18.jpg, The

large blue The large blue (''Phengaris arion'') is a species of butterfly in the family Lycaenidae. The species was first defined in 1758 and first recorded in Britain in 1795. In 1979 the species became mostly extinct in Britain but has been successfully ...

butterfly is an ant mimic and social parasite. File:Eastern Phoebe-nest-Brown-headed-Cowbird-egg.jpg, In

, the host raises the young of another species: here a

's egg in an

Eastern phoebe The eastern phoebe (''Sayornis phoebe'') is a small passerine bird. The genus name ''Sayornis'' is constructed from the specific part of Charles Lucien Bonaparte's name for Say's phoebe, ''Muscicapa saya'', and Ancient Greek ''ornis'', "bird". '' ...

's nest. File:Great Skua (cropped).jpg, The

great skua The great skua (''Stercorarius skua''), sometimes known by the name bonxie in Britain, is a large seabird in the skua family Stercorariidae. It is roughly the size of a herring gull. It mainly eats fish caught at the sea surface or taken fro ...

is a powerful

kleptoparasite Kleptoparasitism (originally spelt clepto-parasitism, meaning "parasitism by theft") is a form of feeding in which one animal deliberately takes food from another. The strategy is Evolutionarily stable strategy, evolutionarily stable when stealin ...

, relentlessly pursuing other seabirds until they disgorge their catches of food. File:Северная церапия (cropped).jpg, The male of the

species ''

'' lives as a tiny sexual parasite permanently attached below the female's body. File:Encarsia perplexa.jpg, '' Encarsia perplexa'' (centre), a parasitoid of citrus blackfly (lower left), is also an adelphoparasite, laying eggs in larvae of its own species

Taxonomic range

Parasitism has an extremely wide taxonomic range, including animals, plants, fungi, protozoans, bacteria, and viruses.

Animals

Parasitism is widespread in the animal kingdom, and has evolved independently from free-living forms hundreds of times. Many types of

including flukes and

have complete life cycles involving two or more hosts. By far the largest group is the parasitoid wasps in the Hymenoptera. The

phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phy ...

and classes with the largest numbers of parasitic species are listed in the table. Numbers are conservative minimum estimates. The columns for Endo- and Ecto-parasitism refer to the definitive host, as documented in the Vertebrate and Invertebrate columns.

Plants

hemiparasite A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All parasitic plants develop a specialized organ called th ...

or ''partial parasite'' such as

derives some of its nutrients from another living plant, whereas a

holoparasite An obligate parasite or holoparasite is a parasitic organism that cannot complete its life-cycle without exploiting a suitable host. If an obligate parasite cannot obtain a host it will fail to reproduce. This is opposed to a facultative parasite, ...

such as ''

Cuscuta ''Cuscuta'' (), commonly known as dodder or amarbel, is a genus of over 201 species of yellow, orange, or red (rarely green) parasitic plants. Formerly treated as the only genus in the family Cuscutaceae, it now is accepted as belonging in the ...

'' derives all of its nutrients from another plant.

Parasitic plant A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All Parasite, parasitic plants develop a specialized organ ...

s make up about one per cent of

angiosperms Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed within a fruit. T ...

and are in almost every

biome A biome () is a distinct geographical region with specific climate, vegetation, and animal life. It consists of a biological community that has formed in response to its physical environment and regional climate. In 1935, Tansley added the ...

in the world. which appeared in Spanish as Chapter 2, pp. 7–27 in: J. A. López-Sáez, P. Catalán and L. Sáez ds. ''Parasitic Plants of the Iberian Peninsula and Balearic Islands''. All these plants have modified roots,

haustoria In botany and mycology, a haustorium (plural haustoria) is a rootlike structure that grows into or around another structure to absorb water or nutrients. For example, in mistletoe or members of the broomrape family, the structure penetrates th ...

, which penetrate the host plants, connecting them to the conductive system—either the

xylem Xylem is one of the two types of transport tissue (biology), tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts o ...

, the

phloem Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is ...

, or both. This provides them with the ability to extract water and nutrients from the host. A parasitic plant is classified depending on where it latches onto the host, either the stem or the root, and the amount of nutrients it requires. Since holoparasites have no

chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...

and therefore cannot make food for themselves by

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

, they are always obligate parasites, deriving all their food from their hosts. Some parasitic plants can locate their

plants by detecting

chemicals A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combin ...

in the air or soil given off by host

shoot Shoot most commonly refers to: * Shoot (botany), an immature plant or portion of a plant * Shooting, the firing of projectile weapons * Photo shoot, a photography session; an event wherein a photographer takes photographs Shoot may also refer t ...

s or

root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often bel ...

s, respectively. About 4,500

of parasitic plant in approximately 20

families Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...

s are known. Species within the ''

Orobanchaceae Orobanchaceae, the broomrapes, is a family (biology), family of mostly parasitic plants of the order (biology), order Lamiales, with about 90 genus, genera and more than 2000 species. Many of these genera (e.g., ''Pedicularis'', ''Rhinanthus'', ...

'' (broomrapes) are among the most economically destructive of all plants. Species of ''

Striga ''Striga'', commonly known as witchweed, is a genus of parasitic plants that occur naturally in parts of Africa, Asia, and Australia. It is currently classified in the family Orobanchaceae, although older classifications place it in the Scrophul ...

'' (witchweeds) are estimated to cost billions of dollars a year in crop yield loss, infesting over 50 million hectares of cultivated land within Sub-Saharan Africa alone. ''Striga'' infects both grasses and grains, including

corn Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout Poaceae, grass that produces cereal grain. It was domesticated by indigenous peoples of Mexico, indigenous peoples in southern Mexico about 9,000 years ago ...

rice Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...

, and

sorghum ''Sorghum bicolor'', commonly called sorghum () and also known as great millet, broomcorn, guinea corn, durra, imphee, jowar, or milo, is a species in the Poaceae, grass genus ''Sorghum (genus), Sorghum'' cultivated for its grain. The grain i ...

, which are among the world's most important food crops. ''

Orobanche ''Orobanche'', commonly known as broomrape, is a genus of almost 200 species of small Parasitic plant, parasitic herbaceous plants, mostly native to the temperate climate, temperate Northern Hemisphere. It is the type genus of the Orobanchaceae ...

'' also threatens a wide range of other important crops, including

peas Pea (''pisum'' in Latin) is a pulse or fodder crop, but the word often refers to the seed or sometimes the pod of this flowering plant species. Peas are eaten as a vegetable. Carl Linnaeus gave the species the scientific name ''Pisum sativum ...

chickpeas The chickpea or chick pea (''Cicer arietinum'') is an annual legume of the family Fabaceae, subfamily Faboideae, cultivated for its edible seeds. Its different types are variously known as gram," Bengal gram, garbanzo, garbanzo bean, or Egypt ...

tomatoes The tomato (, ), ''Solanum lycopersicum'', is a plant whose fruit is an edible berry that is eaten as a vegetable. The tomato is a member of the nightshade family that includes tobacco, potato, and chili peppers. It originated from and was d ...

carrot The carrot ('' Daucus carota'' subsp. ''sativus'') is a root vegetable, typically orange in colour, though heirloom variants including purple, black, red, white, and yellow cultivars exist, all of which are domesticated forms of the wild ...

s, and varieties of

cabbage Cabbage, comprising several cultivars of '' Brassica oleracea'', is a leafy green, red (purple), or white (pale green) biennial plant grown as an annual vegetable crop for its dense-leaved heads. It is descended from the wild cabbage ( ''B.& ...

. Yield loss from ''Orobanche'' can be total; despite extensive research, no method of control has been entirely successful. Many

plants Plants are the eukaryotes that form the kingdom Plantae; they are predominantly photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars f ...

and

exchange carbon and nutrients in mutualistic

mycorrhizal A mycorrhiza (; , mycorrhiza, or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, the plant root system and its surroundings. Mycorrhizae play ...

relationships. Some 400 species of myco-heterotrophic plants, mostly in the tropics, however effectively cheat by taking carbon from a fungus rather than exchanging it for minerals. They have much reduced roots, as they do not need to absorb water from the soil; their stems are slender with few

vascular bundle A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in the stem, which exists in two forms: xylem and phloem. Both these tissues are present in a vascular bundle, which in addition will incl ...

s, and their leaves are reduced to small scales, as they do not photosynthesize. Their seeds are small and numerous, so they appear to rely on being infected by a suitable fungus soon after germinating.

Fungi

Parasitic

derive some or all of their nutritional requirements from plants, other fungi, or animals. Plant pathogenic fungi are classified into three categories depending on their mode of nutrition: biotrophs, hemibiotrophs and necrotrophs. Biotrophic fungi derive nutrients from living plant cells, and during the course of infection they colonise their plant host in such a way as to keep it alive for a maximally long time. One well-known example of a biotrophic pathogen is ''Ustilago maydis'', causative agent of the corn smut disease. Necrotrophic pathogens on the other hand, kill host cells and feed

ally, an example being the root-colonising honey fungi in the genus ''

Armillaria ''Armillaria'' is a genus of fungi that includes the '' A. mellea'' species ('honey fungus') that live on trees and woody shrubs. It includes about 10 species formerly categorized summarily as ''A. mellea''. ''Armillaria'' sp. are long-live ...

.'' Hemibiotrophic pathogens begin their colonising their hosts as biotrophs, and subsequently killing off host cells and feeding as necrotrophs, a phenomenon termed the biotrophy-necrotrophy switch. Pathogenic fungi are well-known causative agents of diseases on animals as well as humans. Fungal infections (

mycosis Fungal infection, also known as mycosis, is a disease caused by fungi. Different types are traditionally divided according to the part of the body affected: superficial, subcutaneous, and systemic. Superficial fungal infections include common ...

) are estimated to kill 1.6 million people each year. One example of a potent fungal animal pathogen are

Microsporidia Microsporidia are a group of spore-forming unicellular parasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore.Franzen, C. (2005). How do Microsporidia inva ...

- obligate intracellular parasitic fungi that largely affect insects, but may also affect vertebrates including humans, causing the intestinal infection

microsporidiosis Microsporidiosis is an opportunistic intestinal infection that causes diarrhea and wasting in immunocompromised individuals (HIV, for example). It results from different species of microsporidia, a group of microbial (unicellular) fungi. In HIV-i ...

Protozoa

Protozoa such as ''

'', ''

'', and ''

Entamoeba ''Entamoeba'' is a genus of Amoebozoa found as internal parasites or commensals of animals. In 1875, Fedor Lösch described the first proven case of amoebic dysentery in St. Petersburg, Russia. He referred to the amoeba he observed microscopica ...

'' are endoparasitic. They cause serious diseases in vertebrates including humans—in these examples, malaria, sleeping sickness, and

—and have complex life cycles.

Bacteria

Many bacteria are parasitic, though they are more generally thought of as

pathogens In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term ...

causing disease. Parasitic bacteria are extremely diverse, and infect their hosts by a variety of routes. To give a few examples, ''

Bacillus anthracis ''Bacillus anthracis'' is a gram-positive and rod-shaped bacterium that causes anthrax, a deadly disease to livestock and, occasionally, to humans. It is the only permanent (obligate) pathogen within the genus ''Bacillus''. Its infection is a ty ...

'', the cause of

anthrax Anthrax is an infection caused by the bacterium '' Bacillus anthracis'' or ''Bacillus cereus'' biovar ''anthracis''. Infection typically occurs by contact with the skin, inhalation, or intestinal absorption. Symptom onset occurs between one ...

, is spread by contact with infected

domestic animal This page gives a list of domesticated animals, also including a list of animals which are or may be currently undergoing the process of domestication and animals that have an extensive relationship with humans beyond simple predation. This includ ...

s; its

spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

s, which can survive for years outside the body, can enter a host through an abrasion or may be inhaled. ''

Borrelia ''Borrelia'' is a genus of bacteria of the spirochete phylum. Several species cause Lyme disease, also called Lyme borreliosis, a zoonotic, vector-borne disease transmitted by ticks. Other species of ''Borrelia'' cause relapsing fever, and are ...

'', the cause of

and

relapsing fever Relapsing fever is a vector-borne disease caused by infection with certain bacteria in the genus '' Borrelia'', which is transmitted through the bites of lice, soft-bodied ticks (genus '' Ornithodoros''), or hard-bodied ticks (Genus Ixodes). ...

, is transmitted by vectors, ticks of the genus ''

Ixodes ''Ixodes'' is a genus of hard-bodied ticks (family Ixodidae). It includes important disease vectors of animals and humans ( tick-borne disease), and some species (notably '' Ixodes holocyclus'') inject toxins that can cause paralysis. Som ...

'', from the diseases' reservoirs in animals such as

deer A deer (: deer) or true deer is a hoofed ruminant ungulate of the family Cervidae (informally the deer family). Cervidae is divided into subfamilies Cervinae (which includes, among others, muntjac, elk (wapiti), red deer, and fallow deer) ...

. ''

Campylobacter jejuni ''Campylobacter jejuni'' is a species of pathogenic bacteria that is commonly associated with poultry, and is also often found in animal feces. This species of microbe is one of the most common causes of food poisoning in Europe and in the US, w ...

'', a cause of

gastroenteritis Gastroenteritis, also known as infectious diarrhea, is an inflammation of the Human gastrointestinal tract, gastrointestinal tract including the stomach and intestine. Symptoms may include diarrhea, vomiting, and abdominal pain. Fever, lack of ...

, is spread by the fecal–oral route from animals, or by eating insufficiently cooked

poultry Poultry () are domesticated birds kept by humans for the purpose of harvesting animal products such as meat, Eggs as food, eggs or feathers. The practice of animal husbandry, raising poultry is known as poultry farming. These birds are most typ ...

, or by contaminated water. ''

Haemophilus influenzae ''Haemophilus influenzae'' (formerly called Pfeiffer's bacillus or ''Bacillus influenzae'') is a Gram-negative, Motility, non-motile, Coccobacillus, coccobacillary, facultative anaerobic organism, facultatively anaerobic, Capnophile, capnophili ...

'', an agent of

bacterial meningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, intense headache, vomiting and neck stiffness and occasionall ...

and respiratory tract infections such as

influenza Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These sympto ...

and

bronchitis Bronchitis is inflammation of the bronchi (large and medium-sized airways) in the lungs that causes coughing. Bronchitis usually begins as an infection in the nose, ears, throat, or sinuses. The infection then makes its way down to the bronchi. ...

, is transmitted by droplet contact. ''

Treponema pallidum ''Treponema pallidum'', formerly known as ''Spirochaeta pallida'', is a Microaerophile, microaerophilic, Gram-negative bacteria, gram-negative, spirochaete bacterium with subspecies that cause the diseases syphilis, bejel (also known as endemic ...

'', the cause of

syphilis Syphilis () is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms depend on the stage it presents: primary, secondary, latent syphilis, latent or tertiary. The prim ...

, is

spread Spread may refer to: Places * Spread, West Virginia Arts, entertainment, and media * ''Spread'' (film), a 2009 film. * ''$pread'', a quarterly magazine by and for sex workers * "Spread", a song by OutKast from their 2003 album ''Speakerboxxx/T ...

sexual activity Human sexual activity, human sexual practice or human sexual behaviour is the manner in which humans experience and express their sexuality. People engage in a variety of sexual acts, ranging from activities done alone (e.g., masturbation) t ...

Viruses

Virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...

es are obligate intracellular parasites, characterised by extremely limited biological function, to the point where, while they are evidently able to infect all other organisms from bacteria and

archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...

to animals, plants and fungi, it is unclear whether they can themselves be described as living. They can be either

RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...

DNA virus A DNA virus is a virus that has a genome made of deoxyribonucleic acid (DNA) that is replicated by a DNA polymerase. They can be divided between those that have two strands of DNA in their genome, called double-stranded DNA (dsDNA) viruses, and t ...

es consisting of a single or double strand of

genetic material Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nucleic aci ...

(

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

, respectively), covered in a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

coat and sometimes a

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

envelope. They thus lack all the usual machinery of the

cell Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...

such as

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s, relying entirely on the host cell's ability to replicate DNA and synthesise proteins. Most viruses are

s, infecting bacteria.

Evolutionary ecology

Parasitism is a major aspect of evolutionary ecology; for example, almost all free-living animals are host to at least one species of parasite. Vertebrates, the best-studied group, are hosts to between 75,000 and 300,000 species of helminths and an uncounted number of parasitic microorganisms. On average, a mammal species hosts four species of nematode, two of trematodes, and two of cestodes. Humans have 342 species of helminth parasites, and 70 species of protozoan parasites. Some three-quarters of the links in

food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Position in the food web, or trophic level, is used in ecology to broadly classify organisms as autotrophs or he ...

s include a parasite, important in regulating host numbers. Perhaps 40 per cent of described species are parasitic.

Fossil record

Parasitism is hard to demonstrate from the

fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

, but holes in the

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

s of several specimens of ''

Tyrannosaurus ''Tyrannosaurus'' () is a genus of large theropod dinosaur. The type species ''Tyrannosaurus rex'' ( meaning 'king' in Latin), often shortened to ''T. rex'' or colloquially t-rex, is one of the best represented theropods. It lived througho ...

'' may have been caused by ''

Trichomonas ''Trichomonas'' is a genus of anaerobic excavate parasites of vertebrates. It was first discovered by Alfred François Donné in 1836 when he found these parasites in the vagina of a patient suffering from vaginitis, an inflammation of the vag ...

''-like parasites. '' Saurophthirus'', the Early Cretaceous

, parasitized

pterosaurs Pterosaurs are an extinct clade of flying reptiles in the Order (biology), order Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 million to 66 million years ago). Pterosau ...

. Eggs that belonged to

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

worms and probably

cyst A cyst is a closed sac, having a distinct envelope and division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (like the manner in which water molecules group together to form a bubb ...

s were found in the Late Triassic

coprolite A coprolite (also known as a coprolith) is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name ...

phytosaur Phytosaurs (Φυτόσαυροι in Greek, meaning 'plant lizard') are an extinct group of large, mostly semiaquatic Late Triassic archosauriform or basal archosaurian reptiles. Phytosaurs belong to the order Phytosauria and are sometimes ref ...

. This rare find in Thailand reveals more about the ecology of prehistoric parasites.

Coevolution

As hosts and parasites evolve together, their relationships often change. When a parasite is in a sole relationship with a host, selection drives the relationship to become more benign, even mutualistic, as the parasite can reproduce for longer if its host lives longer. But where parasites are competing, selection favours the parasite that reproduces fastest, leading to increased virulence. There are thus varied possibilities in

host–parasite coevolution Host–parasite coevolution is a special case of coevolution, where a Host (biology), host and a parasite continually adapt to each other. This can create an evolutionary arms race between them. A more benign possibility is of an evolutionary tra ...

. Evolutionary epidemiology analyses how parasites spread and evolve, whereas Darwinian medicine applies similar evolutionary thinking to non-parasitic diseases like

cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...

and autoimmune conditions.

Long-term partnerships favouring mutualism

Long-term partnerships can lead to a relatively stable relationship tending to

or mutualism, as, all else being equal, it is in the evolutionary interest of the parasite that its host thrives. A parasite may evolve to become less harmful for its host or a host may evolve to cope with the unavoidable presence of a parasite—to the point that the parasite's absence causes the host harm. For example, although animals parasitised by

worms The World Register of Marine Species (WoRMS) is a taxonomic database that aims to provide an authoritative and comprehensive catalogue and list of names of marine organisms. Content The content of the registry is edited and maintained by scien ...

are often clearly harmed, such infections may also reduce the prevalence and effects of

autoimmune In immunology, autoimmunity is the system of immune responses of an organism against its own healthy cells, tissues and other normal body constituents. Any disease resulting from this type of immune response is termed an " autoimmune disease" ...

disorders in animal hosts, including humans. In a more extreme example, some

worms cannot reproduce, or even survive, without infection by ''

Wolbachia ''Wolbachia'' is a genus of gram-negative bacteria infecting many species of arthropods and filarial nematodes. The symbiotic relationship ranges from parasitism to obligate mutualism. It is one of the most common parasitic microbes of arthrop ...

'' bacteria.

Lynn Margulis Lynn Margulis (born Lynn Petra Alexander; March 5, 1938 – November 22, 2011) was an American evolutionary biologist, and was the primary modern proponent for the significance of symbiogenesis, symbiosis in evolution. In particular, Margulis tr ...

and others have argued, following

Peter Kropotkin Pyotr Alexeyevich Kropotkin (9 December 1842 – 8 February 1921) was a Russian anarchist and geographer known as a proponent of anarchist communism. Born into an aristocratic land-owning family, Kropotkin attended the Page Corps and later s ...

's 1902 '' Mutual Aid: A Factor of Evolution'', that natural selection drives relationships from parasitism to mutualism when resources are limited. This process may have been involved in the

symbiogenesis Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibl ...

which formed the

eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

s from an intracellular relationship between

and bacteria, though the sequence of events remains largely undefined.

Competition favouring virulence

Competition between parasites can be expected to favour faster reproducing and therefore more

virulent Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its abilit ...

parasites, by

natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...

. Among competing parasitic insect-killing bacteria of the genera ''

Photorhabdus ''Photorhabdus'' is a genus of bioluminescent, gram-negative bacilli which lives symbiotically within entomopathogenic nematodes, hence the name ''photo'' (which means light producing) and ''rhabdus'' (rod shape). ''Photorhabdus'' is known to be ...

'' and ''

Xenorhabdus ''Xenorhabdus'' is a genus of motile, gram-negative bacteria from the family of the Morganellaceae. All the species of the genus are only known to live in symbiosis with soil entomopathogenic nematodes from the genus '' Steinernema''. Although n ...

'', virulence depended on the relative potency of the antimicrobial

toxin A toxin is a naturally occurring poison produced by metabolic activities of living cells or organisms. They occur especially as proteins, often conjugated. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived ...

s (

bacteriocins Bacteriocins are proteinaceous or peptidic toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strain(s). They are similar to yeast and paramecium killing factors, and are structurally, functionally, and ec ...

) produced by the two strains involved. When only one bacterium could kill the other, the other strain was excluded by the competition. But when

s were infected with bacteria both of which had toxins able to kill the other strain, neither strain was excluded, and their virulence was less than when the insect was infected by a single strain.

Cospeciation

A parasite sometimes undergoes cospeciation with its host, resulting in the pattern described in Fahrenholz's rule, that the phylogenies of the host and parasite come to mirror each other. An example is between the

simian foamy virus ''Simian foamy virus'' (SFV), historically Human foamy virus (HFV), is a species of the genus '' Spumavirus'' that belongs to the family of ''Retroviridae''. It has been identified in a wide variety of primates, including prosimians, New World an ...

(SFV) and its primate hosts. The phylogenies of SFV polymerase and the mitochondrial

cytochrome c oxidase subunit II Cytochrome c oxidase II is a protein in eukaryotes that is encoded by the MT-CO2 gene. Cytochrome c oxidase subunit II, abbreviated COXII, COX2, COII, or MT-CO2, is the second subunit of cytochrome c oxidase. It is also one of the three mitoc ...

from African and Asian primates were found to be closely congruent in branching order and divergence times, implying that the simian foamy viruses cospeciated with Old World primates for at least 30 million years. The presumption of a shared evolutionary history between parasites and hosts can help elucidate how host taxa are related. For instance, there has been a dispute about whether

flamingos Flamingos or flamingoes () are a type of wading bird in the family Phoenicopteridae, which is the only extant family in the order Phoenicopteriformes. There are four flamingo species distributed throughout the Americas (including the Caribbe ...

are more closely related to

storks Storks are large, long-legged, long-necked wading birds with long, stout Beak, bills. They belong to the family (biology), family Ciconiidae, and make up the order Ciconiiformes . Ciconiiformes previously included a number of other families, suc ...

ducks Duck is the common name for numerous species of waterfowl in the family (biology), family Anatidae. Ducks are generally smaller and shorter-necked than swans and goose, geese, which are members of the same family. Divided among several subfam ...

. The fact that flamingos share parasites with ducks and geese was initially taken as evidence that these groups were more closely related to each other than either is to storks. However, evolutionary events such as the duplication, or the extinction of parasite species (without similar events on the host phylogeny) often erode similarities between host and parasite phylogenies. In the case of flamingos, they have similar lice to those of

grebe Grebes () are aquatic diving birds in the order (biology), order Podicipediformes (). Grebes are widely distributed freshwater birds, with some species also found in sea, marine habitats during Bird migration, migration and winter. Most grebes f ...

s. Flamingos and grebes do have a common ancestor, implying cospeciation of birds and lice in these groups. Flamingo lice then switched hosts to ducks, creating the situation which had confused biologists. Parasites infect

sympatric In biology, two closely related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter each other. An initially interbreeding population that splits into two or more distinct spe ...

hosts (those within their same geographical area) more effectively, as has been shown with digenetic trematodes infecting lake snails. This is in line with the

Red Queen hypothesis The Red Queen's hypothesis is a hypothesis in evolutionary biology proposed in 1973, that species must constantly adapt, evolve, and proliferate in order to survive while pitted against ever-evolving opposing species. The hypothesis was intende ...

, which states that interactions between species lead to constant natural selection for coadaptation. Parasites track the locally common hosts' phenotypes, so the parasites are less infective to

allopatric Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...

hosts, those from different geographical regions.

Modifying host behaviour

Some parasites modify host behaviour in order to increase their transmission between hosts, often in relation to predator and prey ( parasite increased trophic transmission). For example, in the

California coastal salt marsh California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

, the fluke '' Euhaplorchis californiensis'' reduces the ability of its

killifish A killifish is any of various oviparous (egg-laying) cyprinodontiform fish, including families Aplocheilidae, Pantanodontidae, Cyprinodontidae, Fundulidae, Nothobranchiidae, Profundulidae, Aphaniidae and Valenciidae. All together, there ar ...

host to avoid predators. This parasite matures in

egret Egrets ( ) are herons, generally long-legged wading birds, that have white or buff plumage, developing fine plumes (usually milky white) during the breeding season. Egrets are not a biologically distinct group from herons and have the same build ...

s, which are more likely to feed on infected killifish than on uninfected fish. Another example is the protozoan ''

Toxoplasma gondii ''Toxoplasma gondii'' () is a species of parasitic alveolate that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-blooded animals, but members of the cat family (felidae) are the only known d ...

'', a parasite that matures in

cats The cat (''Felis catus''), also referred to as the domestic cat or house cat, is a small domesticated carnivorous mammal. It is the only domesticated species of the family Felidae. Advances in archaeology and genetics have shown that the ...

but can be carried by many other

mammals A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...

. Uninfected

rats Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' (pack rats), '' Bandicota'' (bandicoot ...

avoid cat odors, but rats infected with ''T. gondii'' are drawn to this scent, which may increase transmission to feline hosts. The malaria parasite modifies the skin odour of its human hosts, increasing their attractiveness to mosquitoes and hence improving the chance for the parasite to be transmitted. The spider '' Cyclosa argenteoalba'' often have parasitoid wasp larvae attached to them which alter their web-building behavior. Instead of producing their normal sticky spiral shaped webs, they made simplified webs when the parasites were attached. This manipulated behavior lasted longer and was more prominent the longer the parasites were left on the spiders.

Trait loss

Parasites can exploit their hosts to carry out a number of functions that they would otherwise have to carry out for themselves. Parasites which lose those functions then have a selective advantage, as they can divert resources to reproduction. Many insect ectoparasites including bedbugs, batbugs,

and

s have lost their ability to fly, relying instead on their hosts for transport. Trait loss more generally is widespread among parasites. An extreme example is the

myxosporea Myxosporea is a class of microscopic animals, all of whom are parasites. They belong to the Myxozoa clade within Cnidaria. They have a complex life cycle that comprises vegetative forms in two hosts—one an aquatic invertebrate (generally an ann ...

n ''

Henneguya zschokkei ''Henneguya zschokkei'' or ''Henneguya salminicola'' is a species of a myxosporean endoparasite. It afflicts several salmon and trout in the genera ''Oncorhynchus'' and ''Salmo'','' ''where it causes milky flesh or tapioca disease''. H. ...

'', an ectoparasite of fish and the only animal known to have lost the ability to respire aerobically: its cells lack

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

Host defences

Hosts have evolved a variety of defensive measures against their parasites, including physical barriers like the skin of vertebrates, the immune system of mammals, insects actively removing parasites, and defensive chemicals in plants. The evolutionary biologist

W. D. Hamilton William Donald Hamilton (1 August 1936 – 7 March 2000) was a British evolutionary biologist, recognised as one of the most significant evolutionary theorists of the 20th century. Hamilton became known for his theoretical work expounding a ...

suggested that

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...

could have evolved to help to defeat multiple parasites by enabling

genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryot ...

, the shuffling of genes to create varied combinations. Hamilton showed by mathematical modelling that sexual reproduction would be evolutionarily stable in different situations, and that the theory's predictions matched the actual ecology of sexual reproduction. However, there may be a trade-off between

immunocompetence In immunology, immunocompetence is the ability of the body to produce a normal immune response following exposure to an antigen. Immunocompetence is the opposite of immunodeficiency (also known as ''immuno-incompetence'' or being ''immuno-compro ...

and breeding male vertebrate hosts'

s, such as the plumage of

peacock Peafowl is a common name for two bird species of the genus '' Pavo'' and one species of the closely related genus '' Afropavo'' within the tribe Pavonini of the family Phasianidae (the pheasants and their allies). Male peafowl are referred t ...

s and the manes of

lion The lion (''Panthera leo'') is a large Felidae, cat of the genus ''Panthera'', native to Sub-Saharan Africa and India. It has a muscular, broad-chested body (biology), body; a short, rounded head; round ears; and a dark, hairy tuft at the ...

s. This is because the male hormone

testosterone Testosterone is the primary male sex hormone and androgen in Male, males. In humans, testosterone plays a key role in the development of Male reproductive system, male reproductive tissues such as testicles and prostate, as well as promoting se ...

encourages the growth of secondary sex characteristics, favouring such males in

sexual selection Sexual selection is a mechanism of evolution in which members of one sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex ...

, at the price of reducing their immune defences.

Vertebrates

The physical barrier of the tough and often dry and waterproof

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...

of reptiles, birds and mammals keeps invading microorganisms from entering the body.

Human skin The human skin is the outer covering of the body and is the largest organ of the integumentary system. The skin has up to seven layers of ectodermal tissue (biology), tissue guarding Skeletal muscle, muscles, bones, ligaments and organ (anato ...

also secretes

sebum A sebaceous gland or oil gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur ...

, which is toxic to most microorganisms. On the other hand, larger parasites such as

s detect chemicals produced by the skin to locate their hosts when they enter the water. Vertebrate

saliva Saliva (commonly referred as spit or drool) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which ...

and tears contain

lysozyme Lysozyme (, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside hydrolase ...

, an enzyme that breaks down the

cell walls A cell wall is a structural layer that surrounds some cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, and functi ...

of invading bacteria. Should the organism pass the mouth, the

stomach The stomach is a muscular, hollow organ in the upper gastrointestinal tract of Human, humans and many other animals, including several invertebrates. The Ancient Greek name for the stomach is ''gaster'' which is used as ''gastric'' in medical t ...

with its

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

, toxic to most microorganisms, is the next line of defence. Some intestinal parasites have a thick, tough outer coating which is digested slowly or not at all, allowing the parasite to pass through the stomach alive, at which point they enter the intestine and begin the next stage of their life. Once inside the body, parasites must overcome the

immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...

's serum proteins and

pattern recognition receptor Pattern recognition receptors (PRRs) play a crucial role in the proper function of the innate immune system. PRRs are germline-encoded host sensors, which detect molecules typical for the pathogens. They are proteins expressed mainly by cells of th ...

s, intracellular and cellular, that trigger the adaptive immune system's

lymphocyte A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. Lymphocytes include T cells (for cell-mediated and cytotoxic adaptive immunity), B cells (for humoral, antibody-driven adaptive immunity), an ...

s such as

T cell T cells (also known as T lymphocytes) are an important part of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor (TCR) on their cell ...

s and antibody-producing

B cell B cells, also known as B lymphocytes, are a type of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or inserted into the plasm ...

s. These have receptors that recognise parasites.

Insects

Leaf Spot on Oak in Gunnersbury Triangle

Insects often adapt their nests to reduce parasitism. For example, one of the key reasons why the wasp '' Polistes canadensis'' nests across multiple combs, rather than building a single comb like much of the rest of its genus, is to avoid infestation by tineid moths. The tineid moth lays its eggs within the wasps' nests and then these eggs hatch into larvae that can burrow from cell to cell and prey on wasp pupae. Adult wasps attempt to remove and kill moth eggs and larvae by chewing down the edges of cells, coating the cells with an oral secretion that gives the nest a dark brownish appearance.

Plants

Plants respond to parasite attack with a series of chemical defences, such as

polyphenol oxidase Polyphenol oxidase (PPO; also polyphenol oxidase i, chloroplastic), an enzyme involved in fruit browning, is a tetramer that contains four atoms of copper per molecule. PPO may accept monophenols and/or ''o''-diphenols as substrates. The ...

, under the control of the

jasmonic acid Jasmonic acid (JA) is an organic compound found in several plants including jasmine. The molecule is a member of the jasmonate class of plant hormones. It is biosynthesized from linolenic acid by the octadecanoid pathway. It was first isolated ...

-insensitive (JA) and

salicylic acid Salicylic acid is an organic compound with the formula HOC6H4COOH. A colorless (or white), bitter-tasting solid, it is a precursor to and a active metabolite, metabolite of acetylsalicylic acid (aspirin). It is a plant hormone, and has been lis ...

(SA) signalling pathways. The different biochemical pathways are activated by different attacks, and the two pathways can interact positively or negatively. In general, plants can either initiate a specific or a non-specific response. Specific responses involve recognition of a parasite by the plant's cellular receptors, leading to a strong but localised response: defensive chemicals are produced around the area where the parasite was detected, blocking its spread, and avoiding wasting defensive production where it is not needed. Non-specific defensive responses are systemic, meaning that the responses are not confined to an area of the plant, but spread throughout the plant, making them costly in energy. These are effective against a wide range of parasites. When damaged, such as by

lepidoptera Lepidoptera ( ) or lepidopterans is an order (biology), order of winged insects which includes butterflies and moths. About 180,000 species of the Lepidoptera have been described, representing 10% of the total described species of living organ ...

s, leaves of plants including

maize Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout grass that produces cereal grain. It was domesticated by indigenous peoples in southern Mexico about 9,000 years ago from wild teosinte. Native American ...

and

cotton Cotton (), first recorded in ancient India, is a soft, fluffy staple fiber that grows in a boll, or protective case, around the seeds of the cotton plants of the genus '' Gossypium'' in the mallow family Malvaceae. The fiber is almost pure ...

release increased amounts of volatile chemicals such as

terpene Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n ≥ 2. Terpenes are major biosynthetic building blocks. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predomi ...

s that signal they are being attacked; one effect of this is to attract parasitoid wasps, which in turn attack the caterpillars.

Biology and conservation

Ecology and parasitology

Parasitism and parasite evolution were until the twenty-first century studied by

parasitologist Parasitology is the study of parasites, their hosts, and the relationship between them. As a biological discipline, the scope of parasitology is not determined by the organism or environment in question but by their way of life. This means it f ...

s, in a science dominated by medicine, rather than by

ecologist Ecology () is the natural science of the relationships among living organisms and their environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere levels. Ecology overlaps with the closely re ...

s or

evolutionary biologists Evolutionary biology is the subfield of biology that studies the evolutionary processes such as natural selection, common descent, and speciation that produced the diversity of life on Earth. In the 1930s, the discipline of evolutionary bi ...

. Even though parasite-host interactions were plainly ecological and important in evolution, the history of parasitology caused what the evolutionary ecologist Robert Poulin called a "takeover of parasitism by parasitologists", leading ecologists to ignore the area. This was in his opinion "unfortunate", as parasites are "omnipresent agents of natural selection" and significant forces in evolution and ecology. In his view, the long-standing split between the sciences limited the exchange of ideas, with separate conferences and separate journals. The technical languages of ecology and parasitology sometimes involved different meanings for the same words. There were philosophical differences, too: Poulin notes that, influenced by medicine, "many parasitologists accepted that evolution led to a decrease in parasite virulence, whereas modern evolutionary theory would have predicted a greater range of outcomes". Their complex relationships make parasites difficult to place in food webs: a trematode with multiple hosts for its various life cycle stages would occupy many positions in a

simultaneously, and would set up loops of energy flow, confusing the analysis. Further, since nearly every animal has (multiple) parasites, parasites would occupy the top levels of every food web. Parasites can play a role in the proliferation of non-native species. For example, invasive green crabs are minimally affected by native

trematodes Trematoda is a class of flatworms known as trematodes, and commonly as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is a moll ...

on the Eastern Atlantic coast. This helps them outcompete native crabs such as the Atlantic Rock and

Jonah crab The Jonah crab (''Cancer borealis'') is a marine brachyuran crab that inhabits waters along the east coast of North America from Newfoundland to Florida. Jonah crabs possess a rounded, rough-edged carapace with small light spots, and robust claws ...

s. Ecological parasitology can be important to attempts at control, like during the campaign for eradicating the Guinea worm. Even though the parasite was eradicated in all but four countries, the worm began using frogs as an intermediary host before infecting dogs, making control more difficult than it would have been if the relationships had been better understood.

Rationale for conservation

Although parasites are widely considered to be harmful, the eradication of all parasites would not be beneficial. Parasites account for at least half of life's diversity; they perform important ecological roles; and without parasites, organisms might tend to asexual reproduction, diminishing the diversity of traits brought about by sexual reproduction. Parasites provide an opportunity for the transfer of genetic material between species, facilitating evolutionary change. Many parasites require multiple hosts of different species to complete their life cycles and rely on predator-prey or other stable ecological interactions to get from one host to another. The presence of parasites thus indicates that an ecosystem is healthy. An ectoparasite, the California condor louse, ''

Colpocephalum californici ''Colpocephalum californici'', the California condor louse, is an extinct species of chewing louse which parasitized the California condor (''Gymnogyps californianus''). In an example of coextinction, it became extinct when the remaining, Crit ...

'', became a well-known conservation issue. A large and costly captive breeding program was run in the United States to rescue the

California condor The California condor (''Gymnogyps californianus'') is a New World vulture and the largest North American land bird. It became extinct in the wild in 1987 when all remaining wild individuals were captured, but has since been reintroduced to n ...

. It was host to a louse, which lived only on it. Any lice found were "deliberately killed" during the program, to keep the condors in the best possible health. The result was that one species, the condor, was saved and returned to the wild, while another species, the parasite, became extinct. Although parasites are often omitted in depictions of

s, they usually occupy the top position. Parasites can function like

keystone species A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance. The concept was introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in main ...

, reducing the dominance of superior competitors and allowing competing species to co-exist. Aggregated distribution of parasites on hosts

Quantitative ecology

A single parasite species usually has an aggregated distribution across host animals, which means that most hosts carry few parasites, while a few hosts carry the vast majority of parasite individuals. This poses considerable problems for students of parasite ecology, as it renders

parametric statistics Parametric statistics is a branch of statistics which leverages models based on a fixed (finite) set of parameters. Conversely nonparametric statistics does not assume explicit (finite-parametric) mathematical forms for distributions when modeli ...

as commonly used by biologists invalid. Log-transformation of data before the application of parametric test, or the use of

non-parametric statistics Nonparametric statistics is a type of statistical analysis that makes minimal assumptions about the underlying distribution of the data being studied. Often these models are infinite-dimensional, rather than finite dimensional, as in parametric s ...

is recommended by several authors, but this can give rise to further problems, so quantitative parasitology is based on more advanced biostatistical methods.

History

Ancient

Human parasite Human parasites are divided into endoparasites, which cause infection inside the body, and ectoparasites, which cause infection superficially within the skin. Parasites in general are hosts-dependent organisms that obtain nutrients while potential ...

s including roundworms, the

Guinea worm ''Dracunculus medinensis'' (Guinea worm, dragon worm, fiery serpent) is a nematode that causes dracunculiasis, also known as Guinea worm disease. The disease is caused by the female which, at around in length, is among the longest nematodes ...

, threadworms and tapeworms are mentioned in Egyptian papyrus records from 3000 BC onwards; the

Ebers Papyrus The Ebers Papyrus, also known as Papyrus Ebers, is an Egyptian medical papyrus of herbal knowledge dating to (the late Second Intermediate Period or early New Kingdom). Among the oldest and most important medical papyri of Ancient Egypt, it ...

describes

. In

ancient Greece Ancient Greece () was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity (), that comprised a loose collection of culturally and linguistically r ...

, parasites including the bladder worm are described in the

Hippocratic Corpus The Hippocratic Corpus (Latin: ''Corpus Hippocraticum''), or Hippocratic Collection, is a collection of around 60 early Ancient Greek medical works strongly associated with the physician Hippocrates and his teachings. The Hippocratic Corpus cov ...

, while the comic playwright

Aristophanes Aristophanes (; ; ) was an Ancient Greece, Ancient Greek Ancient Greek comedy, comic playwright from Classical Athens, Athens. He wrote in total forty plays, of which eleven survive virtually complete today. The majority of his surviving play ...

called tapeworms "hailstones". The Roman physicians Celsus and

Galen Aelius Galenus or Claudius Galenus (; September 129 – AD), often Anglicization, anglicized as Galen () or Galen of Pergamon, was a Ancient Rome, Roman and Greeks, Greek physician, surgeon, and Philosophy, philosopher. Considered to be one o ...

documented the roundworms ''

Ascaris lumbricoides ''Ascaris lumbricoides'' is a large parasitic worm, parasitic Nematoda, roundworm of the genus ''Ascaris.'' It is the most common parasitic worm in humans. An estimated 807 million–1.2 billion people are infected with ''Ascaris lumbricoides'' ...

'' and ''

Enterobius vermicularis The pinworm (species ''Enterobius vermicularis''), also known as threadworm (in the United Kingdom, Australia and New Zealand) or seatworm, is a parasitic worm. It is a nematode (roundworm) and a common intestinal parasite or helminth, especi ...

''.

Medieval

Francesco Redi Osservazioni intorno agli animali viventi che si trovano negli animali viventi Plate 14

In his '' Canon of Medicine'', completed in 1025, the Persian physician

Avicenna Ibn Sina ( – 22 June 1037), commonly known in the West as Avicenna ( ), was a preeminent philosopher and physician of the Muslim world, flourishing during the Islamic Golden Age, serving in the courts of various Iranian peoples, Iranian ...

recorded human and animal parasites including roundworms, threadworms, the Guinea worm and tapeworms. In his 1397 book ''Traité de l'état, science et pratique de l'art de la Bergerie'' (Account of the state, science and practice of the art of shepherding), wrote the first description of a trematode endoparasite, the sheep liver fluke ''

Fasciola hepatica ''Fasciola hepatica'', also known as the common liver fluke or sheep liver fluke, is a parasitism, parasitic trematode (fluke (flatworm), fluke or flatworm, a type of helminth) of the class (biology), class Trematoda, phylum Platyhelminthes. It ...

''.

Early modern

In the

early modern period The early modern period is a Periodization, historical period that is defined either as part of or as immediately preceding the modern period, with divisions based primarily on the history of Europe and the broader concept of modernity. There i ...

's 1668 book ''Esperienze Intorno alla Generazione degl'Insetti'' (''Experiences of the Generation of Insects''), explicitly described ecto- and endoparasites, illustrating

ticks Ticks are parasitic arachnids of the order Ixodida. They are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, and species, but can become larger when engorged. Ticks a ...

, the larvae of nasal flies of deer, and

. Redi noted that parasites develop from eggs, contradicting the theory of

spontaneous generation Spontaneous generation is a superseded scientific theory that held that living creatures could arise from non-living matter and that such processes were commonplace and regular. It was hypothesized that certain forms, such as fleas, could ...

. In his 1684 book ''Osservazioni intorno agli animali viventi che si trovano negli animali viventi'' (''Observations on Living Animals found in Living Animals''), Redi described and illustrated over 100 parasites including the large roundworm in humans that causes

ascariasis Ascariasis is a disease caused by the parasitic roundworm ''Ascaris lumbricoides''. Infections have no symptoms in more than 85% of cases, especially if the number of worms is small. Symptoms increase with the number of worms present and may in ...

. Redi was the first to name the cysts of ''

Echinococcus granulosus ''Echinococcus granulosus'', also called the hydatid worm or dog tapeworm, is a cyclophyllid cestode that dwells in the small intestine of canids as an adult, but which has important intermediate hosts such as livestock and humans, where it caus ...

'' seen in dogs and sheep as parasitic; a century later, in 1760,

Peter Simon Pallas Peter Simon Pallas Fellow of the Royal Society, FRS FRSE (22 September 1741 – 8 September 1811) was a Prussia, Prussian zoologist, botanist, Ethnography, ethnographer, Exploration, explorer, Geography, geographer, Geology, geologist, Natura ...

correctly suggested that these were the larvae of tapeworms. In 1681,

observed and illustrated the protozoan parasite ''

'', and linked it to "his own loose stools". This was the first protozoan parasite of humans to be seen under a microscope. A few years later, in 1687, the Italian biologists Giovanni Cosimo Bonomo and

Diacinto Cestoni Diacinto (or Giacinto) Cestoni (May 13, 1637 – January 29, 1718) was an Italian naturalist, biologist, botanist, entomologist. Born in Montegiorgio, he was self-taught. He lived and worked at Livorno where he led an apothecary next to the por ...

described

scabies Scabies (; also sometimes known as the seven-year itch) is a contagious human skin infestation by the tiny (0.2–0.45 mm) mite ''Sarcoptes scabiei'', variety ''hominis''. The word is from . The most common symptoms are severe itchiness a ...

as caused by the parasitic mite ''

Sarcoptes scabiei ''Sarcoptes scabiei'' (/sɑː''r''ˈkɒptiːz skeɪˈbiːaɪ/ Traditional English pronunciation of Latin) or the itch mite is a parasite, parasitic mite found in all parts of the world that burrows into skin and causes scabies. Humans become Inf ...

'', marking it as the first disease of humans with a known microscopic causative agent.

Parasitology

Modern

developed in the 19th century with accurate observations and experiments by many researchers and clinicians; the term was first used in 1870. In 1828, James Annersley described

amoebiasis Amoebiasis, or amoebic dysentery, is an infection of the intestines caused by a parasitic amoeba '' Entamoeba histolytica''. Amoebiasis can be present with no, mild, or severe symptoms. Symptoms may include lethargy, loss of weight, colon ...

, protozoal infections of the intestines and the liver, though the pathogen, ''

Entamoeba histolytica ''Entamoeba histolytica'' is an anaerobic organism, anaerobic parasitic amoebozoan, part of the genus ''Entamoeba''. Predominantly infecting humans and other primates causing amoebiasis, ''E. histolytica'' is estimated to infect about 35-50 mil ...

'', was not discovered until 1873 by Friedrich Lösch.

James Paget Sir James Paget, 1st Baronet FRS HFRSE (11 January 1814 – 30 December 1899) (, rhymes with "gadget") was an English surgeon and pathologist who is best remembered for naming Paget's disease and who is considered, together with Rudolf Vir ...

discovered the intestinal nematode ''

Trichinella spiralis ''Trichinella spiralis'' is a viviparous nematode parasite, occurring in rodents, pigs, bears, hyenas and humans, and is responsible for the disease trichinosis. It is sometimes referred to as the "pork worm" due to it being typically encount ...

'' in humans in 1835. James McConnell described the human liver fluke, ''

Clonorchis sinensis ''Clonorchis sinensis'', the Chinese liver fluke, is a liver fluke belonging to the class trematode, Trematoda, phylum Platyhelminthes. It infects fish-eating mammals, including humans. In humans, it infects the common bile duct and gall bladder, ...

'', in 1875. Algernon Thomas and

Rudolf Leuckart Karl Georg Friedrich Rudolf Leuckart (7 October 1822 – 22 February 1898) was a German zoologist born in Helmstedt. He was a pioneer of parasitology research and was widely known for developing a series of illustrated wall charts for use in zo ...

independently made the first discovery of the life cycle of a trematode, the sheep liver fluke, by experiment in 1881–1883. In 1877

Patrick Manson Sir Patrick Manson (3 October 1844 – 9 April 1922) was a Scottish physician who made important discoveries in parasitology, and was a founder of the field of tropical medicine. He graduated from the University of Aberdeen with degrees in Ma ...

discovered the life cycle of the filarial worms that cause

elephantiasis Elephantiasis, often incorrectly called elephantitis, is the enlargement and hardening of limbs or body parts due to tissue swelling (edema). It is characterised by edema, hypertrophy, and fibrosis of skin and subcutaneous tissues, due to obstru ...

transmitted by mosquitoes. Manson further predicted that the

parasite, ''

'', had a mosquito vector, and persuaded

Ronald Ross Sir Ronald Ross (13 May 1857 – 16 September 1932) was a British medical doctor who received the Nobel Prize for Physiology or Medicine in 1902 for his work on the transmission of malaria, becoming the first British Nobel laureate, and the f ...

to investigate. Ross confirmed that the prediction was correct in 1897–1898. At the same time,

Giovanni Battista Grassi Giovanni Battista Grassi (27 March 1854 – 4 May 1925) was an Italian people, Italian physician and zoologist, best known for his pioneering works on parasitology, especially on malariology. He was Professor of Comparative Zoology at the Unive ...

and others described the malaria parasite's life cycle stages in ''

Anopheles ''Anopheles'' () is a genus of mosquito first described by the German entomologist Johann Wilhelm Meigen, J. W. Meigen in 1818, and are known as nail mosquitoes and marsh mosquitoes. Many such mosquitoes are Disease vector, vectors of the paras ...

'' mosquitoes. Ross was controversially awarded the 1902 Nobel prize for his work, while Grassi was not. In 1903, David Bruce identified the protozoan parasite and the

tsetse fly Tsetse ( , or ) (sometimes spelled tzetze; also known as tik-tik flies) are large, biting flies that inhabit much of tropical Africa. Tsetse flies include all the species in the genus ''Glossina'', which are placed in their own family, Gloss ...

vector of

African trypanosomiasis African trypanosomiasis is an insect-borne parasitic infection of humans and other animals. Human African trypanosomiasis (HAT), also known as African sleeping sickness or simply sleeping sickness, is caused by the species ''Trypanosoma bru ...

Vaccine

Given the importance of malaria, with some 220 million people infected annually, many attempts have been made to interrupt its transmission. Various methods of

malaria prophylaxis Malaria prophylaxis is the preventive treatment of malaria. Several malaria vaccines are under development. For pregnant women who are living in malaria endemic areas, routine malaria chemoprevention is recommended. It improves anemia and parasite ...

have been tried including the use of

antimalarial drug Antimalarial medications or simply antimalarials are a type of antiparasitic chemical agent, often naturally derived, that can be used to treat or to prevent malaria, in the latter case, most often aiming at two susceptible target groups, young c ...

s to kill off the parasites in the blood, the eradication of its mosquito vectors with organochlorine and other insecticides, and the development of a malaria vaccine. All of these have proven problematic, with

drug resistance Drug resistance is the reduction in effectiveness of a medication such as an antimicrobial or an antineoplastic in treating a disease or condition. The term is used in the context of resistance that pathogens or cancers have "acquired", that is ...

, insecticide resistance among mosquitoes, and repeated failure of vaccines as the parasite mutates. The first and as of 2015 the only licensed vaccine for any parasitic disease of humans is RTS,S for ''

Plasmodium falciparum ''Plasmodium falciparum'' is a Unicellular organism, unicellular protozoan parasite of humans and is the deadliest species of ''Plasmodium'' that causes malaria in humans. The parasite is transmitted through the bite of a female ''Anopheles'' mos ...

'' malaria.

Biological control

Several groups of parasites, including microbial pathogens and parasitoidal wasps have been used as

biological control Biological control or biocontrol is a method of controlling pests, whether pest animals such as insects and mites, weeds, or pathogens affecting animals or plants by using other organisms. It relies on predation, parasitism, herbivory, or o ...

agents in

agriculture Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created ...

and

horticulture Horticulture (from ) is the art and science of growing fruits, vegetables, flowers, trees, shrubs and ornamental plants. Horticulture is commonly associated with the more professional and technical aspects of plant cultivation on a smaller and mo ...

Resistance

Poulin observes that the widespread

prophylactic Preventive healthcare, or prophylaxis, is the application of healthcare measures to prevent diseases.Hugh R. Leavell and E. Gurney Clark as "the science and art of preventing disease, prolonging life, and promoting physical and mental health a ...

use of anthelmintic drugs in domestic sheep and cattle constitutes a worldwide un

controlled experiment A scientific control is an experiment or observation designed to minimize the effects of variables other than the independent variable (i.e. confounding variables). This increases the reliability of the results, often through a comparison betw ...

in the life-history evolution of their parasites. The outcomes depend on whether the drugs decrease the chance of a helminth larva reaching adulthood. If so, natural selection can be expected to favour the production of eggs at an earlier age. If on the other hand the drugs mainly affects adult

parasitic worm Parasitic worms, also known as helminths, are a polyphyletic group of large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other par ...

s, selection could cause delayed maturity and increased

virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its abili ...

. Such changes appear to be underway: the nematode ''

Teladorsagia circumcincta ''Teladorsagia circumcincta'' is a nematode that is one of the most important parasites of sheep and goats. It was previously known as ''Ostertagia circumcincta'' and is colloquially known as the brown stomach worm. It is common in cool, temperat ...

'' is changing its adult size and reproductive rate in response to drugs.

Cultural significance

THE CRINOLETTA DISFIGURANS AnOldParasite in aNewForm

Classical times

In the

classical era Classical antiquity, also known as the classical era, classical period, classical age, or simply antiquity, is the period of cultural European history between the 8th century BC and the 5th century AD comprising the interwoven civilization ...

, the concept of the parasite was not strictly pejorative: the ''parasitus'' was an accepted role in Roman society, in which a person could live off the hospitality of others, in return for "flattery, simple services, and a willingness to endure humiliation".

Society

Parasitism has a derogatory sense in popular usage. According to the immunologist John Playfair, Playfair is comparing the popular usage to a biologist's view of parasitism, which he calls (heading the same page) "an ancient and respectable view of life". The

cleric

alludes to hyperparasitism in his 1733 poem "On Poetry: A Rhapsody", comparing poets to "vermin" who "teaze and pinch their foes": A 2022 study examined the naming of some 3000 parasite species discovered in the previous two decades. Of those named after scientists, over 80% were named for men, whereas about a third of authors of papers on parasites were women. The study found that the percentage of parasite species named for relatives or friends of the author has risen sharply in the same period.

Fiction

's 1897

novel ''

'', and its many film adaptations, the eponymous

Count Dracula Count Dracula () is the title character of Bram Stoker's 1897 gothic horror novel ''Dracula''. He is considered the prototypical and archetypal vampire in subsequent works of fiction. Aspects of the character are believed by some to have been i ...

is a blood-drinking parasite (a vampire). The critic Laura Otis argues that as a "thief, seducer, creator, and mimic, Dracula is the ultimate parasite. The whole point of vampirism is sucking other people's blood—living at other people's expense." Disgusting and terrifying parasitic alien species are widespread in

, as for instance in

's 1979 film '' Alien''. In one scene, a

Xenomorph The xenomorph (also known as a Xenomorph XX121, ''Internecivus raptus'', ''Plagiarus praepotens'', or simply the alien or the creature)Alien: The Weyland-Yutani Report is a fictional parasitoid, endoparasitoid Extraterrestrials in fiction, extra ...

bursts out of the chest of a dead man, with blood squirting out under high pressure assisted by explosive squibs. Animal organs were used to reinforce the shock effect. The scene was filmed in a single take, and the startled reaction of the actors was genuine. The

entomopathogenic fungus Entomopathogenic fungi are parasitic unicellular or multicellular microorganisms belonging to the kingdom of Fungi, that can infect and seriously disable or kill insects. Pathogenicity for insects is widely distributed in the kingdom of fungi and ...

Cordyceps ''Cordyceps'' is a genus of ascomycete fungi (sac fungi) that includes over 260 species worldwide, many of which are parasitic. Diverse variants of cordyceps have had more than 1,500 years of use in Chinese medicine. Most ''Cordyceps'' specie ...

'' is represented culturally as a deadly threat to the human race. The video game series ''

The Last of Us ''The Last of Us'' is a 2013 action-adventure game developed by Naughty Dog and published by Sony Computer Entertainment. Players control Joel, a smuggler tasked with escorting a teenage girl, Ellie, across a post-apocalyptic United States ...

'' (2013–present) and its

television adaptation An adaptation is a transfer of a work of art from one style, culture or medium to another. Some common examples are: * Film adaptation, a story from another work, adapted into a film (it may be a novel, non-fiction like journalism, autobiography, ...

present ''Cordyceps'' as a parasite of humans, causing a

zombie apocalypse Zombie apocalypse is a subgenre of apocalyptic and post-apocalyptic fiction in which society collapses due to overwhelming swarms of zombies. Usually, only a few individuals or small bands of human survivors are left living. There are many d ...

. Its human hosts initially become violent "infected" beings, before turning into blind zombie "clickers", complete with fruiting bodies growing out from their faces.

Notes

References

Sources

External links

*
Aberystwyth University: Parasitology
��class outline with links to full text articles on parasitism and parasitology.
Division of Parasitic Diseases

Centers for Disease Control and Prevention The Centers for Disease Control and Prevention (CDC) is the National public health institutes, national public health agency of the United States. It is a Federal agencies of the United States, United States federal agency under the United S ...

KSU: Parasitology Research
��parasitology articles and links
Parasitology Resources on the World Wide Web: A Powerful Tool for Infectious Disease Practitioners
(Oxford University Press) {{Authority control Disease ecology Ecology Parasitology

Etymology

Evolutionary strategies

Basic concepts

Major strategies

Parasitic castrators

Directly transmitted

Trophically transmitted

Vector-transmitted

Parasitoids

Micropredators

Transmission strategies

Variations

Hyperparasitism

Social parasitism

Brood parasitism

Kleptoparasitism

Sexual parasitism

Adelphoparasitism

Illustrations

Taxonomic range

Animals

Plants

Fungi

Protozoa

Bacteria

Viruses

Evolutionary ecology

Fossil record

Coevolution

Long-term partnerships favouring mutualism

Competition favouring virulence

Cospeciation

Modifying host behaviour

Trait loss

Host defences

Vertebrates

Insects

Plants

Biology and conservation

Ecology and parasitology

Rationale for conservation

Quantitative ecology

History

Ancient

Medieval

Early modern

Parasitology

Vaccine

Biological control

Resistance

Cultural significance

Classical times

Society

Fiction

See also

Notes

References

Sources

Further reading

External links