In
biology
Biology is the scientific study of life and living organisms. It is a broad natural science that encompasses a wide range of fields and unifying principles that explain the structure, function, growth, History of life, origin, evolution, and ...
, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any
organism
An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
or agent that can produce
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 ...
. A pathogen may also be referred to as an infectious agent, or simply a
germ
Germ or germs may refer to:
Science
* Germ (microorganism), an informal word for a pathogen
* Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually
* Germ layer, a primary layer of cells that forms during embry ...
.
The term ''pathogen'' came into use in the 1880s.
Typically, the term ''pathogen'' is used to describe an ''infectious''
microorganism
A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
or agent, such as a virus, bacterium,
protozoan
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 ...
,
prion
A prion () is a Proteinopathy, misfolded protein that induces misfolding in normal variants of the same protein, leading to cellular death. Prions are responsible for prion diseases, known as transmissible spongiform encephalopathy (TSEs), w ...
,
viroid
Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective eco ...
, or
fungus
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 ...
.
Small animals, such as
helminths
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 ...
and insects, can also cause or
transmit disease. However, these animals are usually referred to as
parasite
Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...
s rather than pathogens.
The scientific study of microscopic organisms, including microscopic pathogenic organisms, is called
microbiology
Microbiology () is the branches of science, scientific study of microorganisms, those being of unicellular organism, unicellular (single-celled), multicellular organism, multicellular (consisting of complex cells), or non-cellular life, acellula ...
, while
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 ...
refers to the scientific study of parasites and the organisms that host them.
There are several pathways through which pathogens can invade a host. The principal pathways have different episodic time frames, but
soil
Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...
has the longest or most persistent potential for harboring a pathogen.
Diseases in humans that are caused by infectious agents are known as pathogenic diseases. Not all diseases are caused by pathogens, such as
black lung from exposure to the pollutant
coal dust
Coal dust is a fine-powdered form of coal which is created by the crushing, grinding, or pulverizer, pulverization of coal rock. Because of the brittle nature of coal, coal dust can be created by mining, transporting, or mechanically handling it. ...
,
genetic disorder
A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosome abnormality. Although polygenic disorders ...
s like
sickle cell disease
Sickle cell disease (SCD), also simply called sickle cell, is a group of inherited Hemoglobinopathy, haemoglobin-related blood disorders. The most common type is known as sickle cell anemia. Sickle cell anemia results in an abnormality in the ...
, and
autoimmune disease
An autoimmune disease is a condition that results from an anomalous response of the adaptive immune system, wherein it mistakenly targets and attacks healthy, functioning parts of the body as if they were foreign organisms. It is estimated tha ...
s like
lupus
Lupus, formally called systemic lupus erythematosus (SLE), is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue in many parts of the body. Symptoms vary among people and may be mild to severe. Common ...
.
Pathogenicity
Pathogenicity is the potential disease-causing capacity of pathogens, involving a combination of
infectivity
In epidemiology, infectivity is the ability of a 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 refer ...
(pathogen's ability to infect hosts) and
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 ...
(severity of host disease).
Koch's postulates
Koch's postulates ( ) are four criteria designed to establish a causality, causal relationship between a microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884, based on earlier concepts described by ...
are used to establish causal relationships between microbial pathogens and diseases. Whereas
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 occasion ...
can be caused by a variety of bacterial, viral, fungal, and parasitic pathogens,
cholera
Cholera () is an infection of the small intestine by some Strain (biology), strains of the Bacteria, bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea last ...
is only caused by some strains of ''
Vibrio cholerae
''Vibrio cholerae'' is a species of Gram-negative bacteria, Gram-negative, Facultative anaerobic organism, facultative anaerobe and Vibrio, comma-shaped bacteria. The bacteria naturally live in Brackish water, brackish or saltwater where they att ...
''. Additionally, some pathogens may only cause disease in hosts with an
immunodeficiency
Immunodeficiency, also known as immunocompromise, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that aff ...
. These
opportunistic infection
An opportunistic infection is an infection that occurs most commonly in individuals with an immunodeficiency disorder and acts more severe on those with a weakened immune system. These types of infections are considered serious and can be caused b ...
s often involve
hospital-acquired infection
A hospital-acquired infection, also known as a nosocomial infection (from the Greek , meaning "hospital"), is an infection that is acquired in a hospital or other health care, healthcare facility. To emphasize both hospital and nonhospital sett ...
s among patients already combating another condition.
Infectivity involves
pathogen transmission
In medicine, public health, and biology, transmission is the passing of a pathogen causing communicable disease from an infected host individual or group to a particular individual or group, regardless of whether the other individual was previo ...
through direct contact with the bodily fluids or airborne droplets of infected hosts, indirect contact involving contaminated areas/items, or transfer by living
vectors like
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 ...
s 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. The
basic reproduction number
In epidemiology, the basic reproduction number, or basic reproductive number (sometimes called basic reproduction ratio or basic reproductive rate), denoted R_0 (pronounced ''R nought'' or ''R zero''), of an infection is the expected number ...
of an infection is the expected number of subsequent cases it is likely to cause through transmission.
Virulence involves pathogens extracting host nutrients for their survival, evading host immune systems by producing
microbial toxin
Microbial toxins are toxins produced by micro-organisms, including bacteria, fungi, protozoa, dinoflagellates, and viruses. Many microbial toxins promote infection and disease by directly damaging host tissues and by disabling the immune system. ...
s and causing
immunosuppression
Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse react ...
.
Optimal virulence describes a theorized equilibrium between a pathogen spreading to additional hosts to parasitize resources, while lowering their virulence to keep hosts living for
vertical transmission
Vertical transmission of symbionts is the transfer of a microbial symbiont from the parent directly to the offspring. Many metazoan species carry symbiotic bacteria which play a mutualistic, commensal, or parasitic role. A symbiont is acq ...
to their offspring.
Types
Algae
Algae include single-celled
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 that are generally non-pathogenic. Green algae from the genus ''
Prototheca
''Prototheca'' is a genus of algae in the family Chlorellaceae. While this genus is a member of the green algae, all ''Prototheca'' no longer have chloroplasts and therefore their photosynthetic ability. Some species can cause protothecosis in ...
'' lack chlorophyll and are known to cause the disease
protothecosis
Protothecosis, otherwise known as Algaemia, is a disease found in dogs, cats, cattle, and humans caused by a type of green alga known as '' Prototheca'' that lacks chlorophyll and enters the human or animal bloodstream. It and its close relative ...
in humans, dogs, cats, and cattle, typically involving the soil-associated species ''
Prototheca wickerhamii
''Prototheca wickerhamii'' is a ubiquitous green alga that does not have chlorophyll. It is widely present in the environment but is a rare cause of opportunistic infection in humans ( protothecosis).
Biology
''P. wickerhamii'' has a round to el ...
''.
Bacteria
Bacteria are single-celled
prokaryote
A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
s that range in size from 0.15 and 700 μM. While the vast majority are either harmless or beneficial to their hosts, such as members of the
human gut microbiome that support digestion, a small percentage are pathogenic and cause infectious diseases. Bacterial
virulence factor
Virulence factors (preferably known as pathogenicity factors or effectors in botany) are cellular structures, molecules and regulatory systems that enable microbial pathogens (bacteria, viruses, fungi, and protozoa) to achieve the following:
* c ...
s include adherence factors to attach to host cells, invasion factors supporting entry into host cells,
capsules to prevent
opsonization
Opsonins are extracellular proteins that, when bound to substances or cells, induce phagocytes to phagocytose the substances or cells with the opsonins bound. Thus, opsonins act as tags to label things in the body that should be phagocytosed (i.e. ...
and
phagocytosis
Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...
, toxins, and
siderophore
Siderophores (Greek: "iron carrier") are small, high-affinity iron- chelating compounds that are secreted by microorganisms such as bacteria and fungi. They help the organism accumulate iron. Although a widening range of siderophore functions is n ...
s to acquire iron.

The bacterial disease
tuberculosis
Tuberculosis (TB), also known colloquially as the "white death", or historically as consumption, is a contagious disease usually caused by ''Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can al ...
, primarily caused by ''
Mycobacterium tuberculosis
''Mycobacterium tuberculosis'' (M. tb), also known as Koch's bacillus, is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of tuberculosis.
First discovered in 1882 by Robert Koch, ''M. tuberculosis'' ha ...
'', has one of the highest
disease burden
Disease burden is the impact of a health problem as measured by financial cost, mortality, morbidity, or other indicators. It is often quantified in terms of quality-adjusted life years (QALYs) or disability-adjusted life years (DALYs). Both ...
s, killing 1.6 million people in 2021, mostly in Africa and Southeast Asia.
Bacterial pneumonia
Bacterial pneumonia is a type of pneumonia caused by bacterial infection.
Types
Gram-positive
'' Streptococcus pneumoniae'' () is the most common bacterial cause of pneumonia in all age groups except newborn infants. ''Streptococcus pneumoniae ...
is primarily caused by ''
Streptococcus pneumoniae
''Streptococcus pneumoniae'', or pneumococcus, is a Gram-positive, spherical bacteria, hemolysis (microbiology), alpha-hemolytic member of the genus ''Streptococcus''. ''S. pneumoniae'' cells are usually found in pairs (diplococci) and do not f ...
'', ''
Staphylococcus aureus
''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often posi ...
'', ''
Klebsiella pneumoniae
''Klebsiella pneumoniae'' is a Gram-negative, non-motile, encapsulated, lactose- fermenting, facultative anaerobic, rod-shaped bacterium. It appears as a mucoid lactose fermenter on MacConkey agar.
Although found in the normal flora of the mo ...
'', and ''
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 ...
''. Foodborne illnesses typically involve ''
Campylobacter
''Campylobacter'' is a type of bacteria that can cause a diarrheal disease in people. Its name means "curved bacteria", as the germ typically appears in a comma or "s" shape. According to its scientific classification, it is a genus of gram-negat ...
'', ''
Clostridium perfringens
''Clostridium perfringens'' (formerly known as ''C. welchii'', or ''Bacillus welchii'') is a Gram-positive, bacillus (rod-shaped), anaerobic, spore-forming pathogenic bacterium of the genus '' Clostridium''. ''C. perfringens'' is ever-present ...
'', ''
Escherichia coli
''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...
'', ''
Listeria monocytogenes
''Listeria monocytogenes'' is the species of pathogenic bacteria that causes the infection listeriosis. It is a facultative anaerobic bacterium, capable of surviving in the presence or absence of oxygen. It can grow and reproduce inside the ho ...
'', and ''
Salmonella
''Salmonella'' is a genus of bacillus (shape), rod-shaped, (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two known species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' ...
''. Other infectious diseases caused by pathogenic bacteria include
tetanus
Tetanus (), also known as lockjaw, is a bacterial infection caused by ''Clostridium tetani'' and characterized by muscle spasms. In the most common type, the spasms begin in the jaw and then progress to the rest of the body. Each spasm usually l ...
,
typhoid fever
Typhoid fever, also known simply as typhoid, is a disease caused by '' Salmonella enterica'' serotype Typhi bacteria, also called ''Salmonella'' Typhi. Symptoms vary from mild to severe, and usually begin six to 30 days after exposure. Often th ...
,
diphtheria
Diphtheria is an infection caused by the bacteria, bacterium ''Corynebacterium diphtheriae''. Most infections are asymptomatic or have a mild Course (medicine), clinical course, but in some outbreaks, the mortality rate approaches 10%. Signs a ...
, and
leprosy
Leprosy, also known as Hansen's disease (HD), is a Chronic condition, long-term infection by the bacteria ''Mycobacterium leprae'' or ''Mycobacterium lepromatosis''. Infection can lead to damage of the Peripheral nervous system, nerves, respir ...
.
Fungi
Fungi are eukaryotic organisms that can function as pathogens. There are approximately 300 known fungi that are pathogenic to humans, including ''
Candida albicans
''Candida albicans'' is an opportunistic pathogenic yeast that is a common member of the human gut flora. It can also survive outside the human body. It is detected in the gastrointestinal tract and mouth in 40–60% of healthy adults. It is usu ...
'', which is the most common cause of
thrush
Thrush may refer to:
Birds
* Thrush (bird), any of the birds in the family Turdidae
** List of thrush species
* Antthrushes, the Formicariidae family of birds
* Dohrn's warbler, or Dohrn's thrush-babbler, a species ''Sylvia dohrni'' in the famil ...
, and ''
Cryptococcus neoformans
''Cryptococcus neoformans'' is an encapsulated basidiomycetous yeast belonging to the class Tremellomycetes and an obligate aerobe that can live in both plants and animals. Its teleomorph is a filamentous fungus, formerly referred to ''Filob ...
'', which can cause a severe form of
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 occasion ...
. Typical fungal spores are 4.7 μm long or smaller.
Prions
Prion
A prion () is a Proteinopathy, misfolded protein that induces misfolding in normal variants of the same protein, leading to cellular death. Prions are responsible for prion diseases, known as transmissible spongiform encephalopathy (TSEs), w ...
s are misfolded proteins that transmit their abnormal folding pattern to other copies of the protein without using
nucleic acid
Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nuclei ...
s. Besides obtaining prions from others, these misfolded proteins arise from genetic differences, either due to family history or sporadic mutations. Plants uptake prions from contaminated soil and transport them into their stem and leaves, potentially transmitting the prions to
herbivorous animals
A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat n ...
. Additionally, wood, rocks, plastic, glass, cement, stainless steel, and aluminum have been shown binding, retaining, and releasing prions, showcasing that the proteins resist environmental degradation.
Prions are best known for causing
transmissible spongiform encephalopathy
Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are a group of progressive, incurable, and fatal conditions that are associated with the prion hypothesis and affect the brain and nervous system of many animals, in ...
(TSE) diseases like
Creutzfeldt–Jakob disease
Creutzfeldt–Jakob disease (CJD) is an incurable, always fatal neurodegenerative disease belonging to the transmissible spongiform encephalopathy (TSE) group. Early symptoms include memory problems, behavioral changes, poor coordination, visu ...
(CJD),
variant Creutzfeldt–Jakob disease
Variant Creutzfeldt–Jakob disease (vCJD), formerly known as new variant Creutzfeldt–Jakob disease (nvCJD) and referred to colloquially as "mad cow disease" or "human mad cow disease" to distinguish it from its BSE counterpart, is a fatal ...
(vCJD),
Gerstmann–Sträussler–Scheinker syndrome (GSS),
fatal familial insomnia
Fatal insomnia is an extremely rare neurodegenerative prion disease that results in trouble sleeping as its hallmark symptom. The majority of cases are familial (fatal familial insomnia FI, stemming from a mutation in the ''PRNP'' gene, wit ...
(FFI), and
kuru in humans.
While prions are typically viewed as pathogens that cause protein
amyloid
Amyloids are aggregates of proteins characterised by a fibrillar morphology of typically 7–13 nm in diameter, a β-sheet secondary structure (known as cross-β) and ability to be stained by particular dyes, such as Congo red. In the human ...
fibers to accumulate into neurodegenerative plaques,
Susan Lindquist led research showing that yeast use prions to pass on evolutionarily beneficial traits.
Viroids
Not to be confused with
virusoids or viruses, viroids are the smallest known infectious pathogens. Viroids are small single-stranded, circular RNA that are only known to cause plant diseases, such as the
potato spindle tuber viroid
The potato spindle tuber viroid (PSTVd, ''Pospiviroid fusituberis'') was the first viroid to be identified. that affects various agricultural crops. Viroid RNA is not protected by a protein coat, and it does not encode any proteins, only acting as a
ribozyme
Ribozymes (ribonucleic acid enzymes) are RNA molecules that have the ability to Catalysis, catalyze specific biochemical reactions, including RNA splicing in gene expression, similar to the action of protein enzymes. The 1982 discovery of ribozy ...
to catalyze other biochemical reactions.
Viruses
Viruses are generally between 20–200 nm in diameter. For survival and replication, viruses inject their genome into host cells, insert those genes into the host genome, and hijack the host's machinery to produce hundreds of new viruses until the cell bursts open to release them for additional infections. The
lytic cycle
The lytic cycle ( ) is one of the two cycles of viral reproduction (referring to bacterial viruses or bacteriophages), the other being the lysogenic cycle. The lytic cycle results in the destruction of the infected cell and its membrane. Bacter ...
describes this active state of rapidly killing hosts, while the
lysogenic cycle
Lysogeny, or the lysogenic cycle, is one of two cycles of Virus, viral reproduction (the lytic cycle being the other). Lysogeny is characterized by integration of the bacteriophage nucleic acid into the host Bacteria, bacterium's genome or form ...
describes potentially hundreds of years of dormancy while integrated in the host genome. Alongside the taxonomy organized by the
International Committee on Taxonomy of Viruses
The International Committee on Taxonomy of Viruses (ICTV) authorizes and organizes the taxonomic classification of and the nomenclature for viruses. The ICTV develops a universal taxonomic scheme for viruses, and thus has the means to appropri ...
(ICTV), the
Baltimore classification
Baltimore classification is a system used to classify viruses based on their manner of messenger RNA (mRNA) synthesis. By organizing viruses based on their manner of mRNA production, it is possible to study viruses that behave similarly as a disti ...
separates viruses by seven classes of
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein.
mRNA is ...
production:
* I: dsDNA viruses (e.g.,
Adenoviruses
Adenoviruses (members of the family ''Adenoviridae'') are medium-sized (90–100 nm), nonenveloped (without an outer lipid bilayer) viruses with an icosahedral nucleocapsid containing a double-stranded DNA genome. Their name derives from the ...
,
Herpesviruses, and
Poxviruses
''Poxviridae'' is a family of double-stranded DNA viruses. Vertebrates and arthropods serve as natural hosts. The family contains 22 genera that are assigned to two subfamilies: ''Chordopoxvirinae'' and ''Entomopoxvirinae''. ''Entomopoxvirinae'' ...
) cause
herpes
Herpes simplex, often known simply as herpes, is a viral infection caused by the herpes simplex virus. Herpes infections are categorized by the area of the body that is infected. The two major types of herpes are oral herpes and genital herp ...
,
chickenpox
Chickenpox, also known as varicella ( ), is a highly contagious disease caused by varicella zoster virus (VZV), a member of the herpesvirus family. The disease results in a characteristic skin rash that forms small, itchy blisters, which ...
, and
smallpox
Smallpox was an infectious disease caused by Variola virus (often called Smallpox virus), which belongs to the genus '' Orthopoxvirus''. The last naturally occurring case was diagnosed in October 1977, and the World Health Organization (W ...
* II: ssDNA viruses (+ strand or "sense") DNA (e.g.,
Parvoviruses
Parvoviruses are a family of animal viruses that constitute the family ''Parvoviridae''. They have linear, single-stranded DNA (ssDNA) genomes that typically contain two genes encoding for a replication initiator protein, called NS1, and the p ...
) include
parvovirus B19
Parvovirus B19, also called B19 virus (B19V), Human parvovirus B19, or sometimes erythrovirus B19, is a human virus in the family ''Parvoviridae'', genus ''Erythroparvovirus''. It measures only 23–26 nm in diameter. The virus is assigned ...
* III: dsRNA viruses (e.g.,
Reoviruses
''Sedoreoviridae'' (''sedo'' = smooth) is a family of the ''Reovirales'' order of viruses. Viruses in this family are distinguished by the absence of a turreted protein on the inner capsid to produce a smooth surface.
Taxonomy
The family contain ...
) include
rotavirus
Rotaviruses are the most common cause of diarrhea, diarrhoeal disease among infants and young children. Nearly every child in the world is infected with a rotavirus at least once by the age of five. Immunity (medical), Immunity develops with ...
es
* IV: (+)ssRNA viruses (+ strand or sense) RNA (e.g.,
Coronavirus
Coronaviruses are a group of related RNA viruses that cause diseases in mammals and birds. In humans and birds, they cause respiratory tract infections that can range from mild to lethal. Mild illnesses in humans include some cases of the comm ...
es,
Picornavirus
Picornaviruses are a group of related Viral envelope, nonenveloped RNA viruses which infect vertebrates including fish, mammals, and birds. They are viruses that represent a large family of small, Positive-sense single-stranded RNA virus, positi ...
es, and
Togaviruses) cause
COVID-19
Coronavirus disease 2019 (COVID-19) is a contagious disease caused by the coronavirus SARS-CoV-2. In January 2020, the disease spread worldwide, resulting in the COVID-19 pandemic.
The symptoms of COVID‑19 can vary but often include fever ...
,
dengue fever
Dengue fever is a mosquito-borne disease caused by dengue virus, prevalent in tropical and subtropical areas. Asymptomatic infections are uncommon, mild cases happen frequently; if symptoms appear, they typically begin 3 to 14 days after i ...
,
Hepatitis A
Hepatitis A is an infectious liver disease caused by Hepatitis A virus (HAV); it is a type of viral hepatitis. Many cases have few or no symptoms, especially in the young. The time between infection and symptoms, in those who develop them, is ...
,
Hepatitis C
Hepatitis C is an infectious disease caused by the hepatitis C virus (HCV) that primarily affects the liver; it is a type of viral hepatitis. During the initial infection period, people often have mild or no symptoms. Early symptoms can include ...
,
rubella
Rubella, also known as German measles or three-day measles, is an infection caused by the rubella virus. This disease is often mild, with half of people not realizing that they are infected. A rash may start around two weeks after exposure and ...
, and
yellow fever
* V: (−)ssRNA viruses (− strand or antisense) RNA (e.g.,
Orthomyxoviruses and
Rhabdoviruses) cause
ebola
Ebola, also known as Ebola virus disease (EVD) and Ebola hemorrhagic fever (EHF), is a viral hemorrhagic fever in humans and other primates, caused by ebolaviruses. Symptoms typically start anywhere between two days and three weeks after in ...
,
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 ...
,
measles
Measles (probably from Middle Dutch or Middle High German ''masel(e)'', meaning "blemish, blood blister") is a highly contagious, Vaccine-preventable diseases, vaccine-preventable infectious disease caused by Measles morbillivirus, measles v ...
,
mumps
MUMPS ("Massachusetts General Hospital Utility Multi-Programming System"), or M, is an imperative, high-level programming language with an integrated transaction processing key–value database. It was originally developed at Massachusetts Gen ...
, and
rabies
Rabies is a viral disease that causes encephalitis in humans and other mammals. It was historically referred to as hydrophobia ("fear of water") because its victims panic when offered liquids to drink. Early symptoms can include fever and abn ...
* VI: ssRNA-RT viruses (+ strand or sense) RNA with DNA intermediate in life-cycle (e.g.,
Retrovirus
A retrovirus is a type of virus that inserts a DNA copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. After invading a host cell's cytoplasm, the virus uses its own reverse transcriptase e ...
es) cause
HIV/AIDS
The HIV, human immunodeficiency virus (HIV) is a retrovirus that attacks the immune system. Without treatment, it can lead to a spectrum of conditions including acquired immunodeficiency syndrome (AIDS). It is a Preventive healthcare, pr ...
* VII: dsDNA-RT viruses DNA with RNA intermediate in life-cycle (e.g.,
Hepadnaviruses) cause
Hepatitis B
Hepatitis B is an infectious disease caused by the '' hepatitis B virus'' (HBV) that affects the liver; it is a type of viral hepatitis. It can cause both acute and chronic infection.
Many people have no symptoms during an initial infection. ...
Other parasites

Protozoans are single-celled eukaryotes that feed on microorganisms and organic tissues. Many protozoans act as pathogenic parasites to cause diseases like
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 ...
,
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 ...
,
giardiasis
Giardiasis is a parasitic disease caused by the protist enteropathogen ''Giardia duodenalis'' (also known as ''G. lamblia'' and ''G. intestinalis''), especially common in children and travellers. Infected individuals experience steatorrhea, a typ ...
,
toxoplasmosis
Toxoplasmosis is a parasitic disease caused by ''Toxoplasma gondii'', an apicomplexan. Infections with toxoplasmosis are associated with a variety of neuropsychiatric and behavioral conditions. Occasionally, people may have a few weeks or month ...
,
cryptosporidiosis
Cryptosporidiosis, sometimes informally called crypto, is a parasitic disease caused by ''Cryptosporidium'', a genus of protozoan parasites in the phylum Apicomplexa. It affects the ileum, distal small intestine and can affect the respiratory tr ...
,
trichomoniasis
Trichomoniasis (trich) is an infectious disease caused by the parasite ''Trichomonas vaginalis''. About 70% of affected people do not have symptoms when infected. When symptoms occur, they typically begin 5 to 28 days after exposure. Symptoms ca ...
,
Chagas disease
Chagas disease, also known as American trypanosomiasis, is a tropical parasitic disease caused by ''Trypanosoma cruzi''. It is spread mostly by insects in the subfamily Triatominae, known as "kissing bugs". The symptoms change throughout the ...
,
leishmaniasis
Leishmaniasis is a wide array of clinical manifestations caused by protozoal parasites of the Trypanosomatida genus ''Leishmania''. It is generally spread through the bite of Phlebotominae, phlebotomine Sandfly, sandflies, ''Phlebotomus'' an ...
,
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 ...
(sleeping sickness),
''Acanthamoeba'' keratitis, and
primary amoebic meningoencephalitis
Naegleriasis, also known as primary amoebic meningoencephalitis (PAM), is an almost invariably fatal infection of the brain by the free-living protozoan '' Naegleria fowleri''. Symptoms include headache, fever, nausea, vomiting, a stiff neck, ...
(naegleriasis).
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 (helminths) are macroparasites that can be seen by the naked eye. Worms live and feed in their living host, acquiring nutrients and shelter in the digestive tract or bloodstream of their host. They also manipulate the host's immune system by secreting immunomodulatory products which allows them to live in their host for years.
Helminthiasis
Helminthiasis, also known as worm infection, is any macroparasite, macroparasitic disease of humans and other animals in which a part of the body is infected with parasitism, parasitic worms, known as helminths. There are numerous species of the ...
is the generalized term for parasitic worm infections, which typically involve
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 ...
,
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 ...
, and
flatworms
Platyhelminthes (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates commonly called ...
.
Pathogen hosts
Bacteria
While bacteria are typically viewed as pathogens, they serve as hosts to
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 ...
viruses (commonly known as phages). The bacteriophage life cycle involves the viruses injecting their genome into bacterial cells, inserting those genes into the bacterial genome, and hijacking the bacteria's machinery to produce hundreds of new phages until the cell bursts open to release them for additional infections. Typically, bacteriophages are only capable of infecting a specific species or strain.
''
Streptococcus pyogenes
''Streptococcus pyogenes'' is a species of Gram-positive, aerotolerant bacteria in the genus '' Streptococcus''. These bacteria are extracellular, and made up of non-motile and non-sporing cocci (round cells) that tend to link in chains. They ...
'' uses a
Cas9
Cas9 (CRISPR associated protein 9, formerly called Cas5, Csn1, or Csx12) is a 160 dalton (unit), kilodalton protein which plays a vital role in the immunological defense of certain bacteria against DNA viruses and plasmids, and is heavily utili ...
nuclease
In biochemistry, a nuclease (also archaically known as nucleodepolymerase or polynucleotidase) is an enzyme capable of cleaving the phosphodiester bonds that link nucleotides together to form nucleic acids. Nucleases variously affect single and ...
to cleave foreign DNA matching the Clustered Regularly Interspaced Short Palindromic Repeats (
CRISPR
CRISPR (; acronym of clustered regularly interspaced short palindromic repeats) is a family of DNA sequences found in the genomes of prokaryotic organisms such as bacteria and archaea. Each sequence within an individual prokaryotic CRISPR is d ...
) associated with bacteriophages, removing the viral genes to avoid infection. This mechanism has been modified for artificial
CRISPR gene editing
CRISPR gene editing (; pronounced like "crisper"; an abbreviation for "clustered regularly interspaced short palindromic repeats") is a genetic engineering technique in molecular biology by which the genomes of living organisms may be modified. ...
.
Plants
Plants can play host to a wide range of pathogen types, including viruses, bacteria, fungi, nematodes, and even other plants. Notable plant viruses include the
papaya ringspot virus
''Papaya ringspot virus'' (PRSV) is a pathogenic plant virus in the genus ''Potyvirus'' and the virus family ''Potyviridae'' which primarily infects the papaya tree.
The virus is a non-enveloped, flexuous rod-shaped particle that is between 760& ...
, which has caused millions of dollars of damage to farmers in Hawaii and Southeast Asia, and the
tobacco mosaic virus
Tobacco mosaic virus (TMV) is a positive-sense single-stranded RNA virus species in the genus '' Tobamovirus'' that infects a wide range of plants, especially tobacco and other members of the family Solanaceae. The infection causes characteris ...
which caused scientist
Martinus Beijerinck
Martinus Willem Beijerinck (, 16 March 1851 – 1 January 1931) was a Dutch microbiologist and botanist who was one of the founders of virology and environmental microbiology. He is credited with the co-discovery of viruses
A virus i ...
to coin the term "virus" in 1898. Bacterial
plant pathogens
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 ...
cause leaf spots, blight, and rot in many plant species.
The most common bacterial pathogens for plants are ''
Pseudomonas syringae
''Pseudomonas syringae'' is a rod-shaped, Gram-negative bacterium with polar flagella. As a plant pathology, plant pathogen, it can infect a wide range of species, and exists as over 50 different pathovars, all of which are available to research ...
'' and ''
Ralstonia solanacearum
''Ralstonia solanacearum'' is an aerobic non-spore-forming, Gram-negative, plant pathogenic bacterium. ''R. solanacearum'' is soil-borne and motile with a polar flagellar tuft. It colonises the xylem, causing bacterial wilt in a very wide ran ...
'', which cause leaf browning and other issues in potatoes, tomatoes, and bananas.
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 ...
are another major pathogen type for plants. They can cause a wide variety of issues such as shorter plant height, growths or pits on tree trunks, root or seed rot, and leaf spots. Common and serious plant fungi include the
rice blast fungus,
Dutch elm disease,
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 ...
and the
black knot
''Dibotryon morbosum'' or ''Apiosporina morbosa'' is a plant pathogen, which is the causal agent of black knot. It affects members of the ''Prunus'' genus such as cherry, plum, apricot, and chokecherry trees in North America. The disease produce ...
and
brown rot diseases of cherries, plums, and peaches. It is estimated that pathogenic fungi alone cause up to a 65% reduction in crop yield.
Overall, plants have a wide array of pathogens and it has been estimated that only 3% of the disease caused by plant pathogens can be managed.
Animals
Animals often get infected with many of the same or similar pathogens as humans including prions, viruses, bacteria, and fungi. While wild animals often get illnesses, the larger danger is for livestock animals. It is estimated that in rural settings, 90% or more of livestock deaths can be attributed to pathogens. Animal transmissible spongiform encephalopathy (TSEs) involving prions include
bovine spongiform encephalopathy
Bovine spongiform encephalopathy (BSE), commonly known as mad cow disease, is an incurable and always fatal neurodegenerative disease of cattle. Symptoms include abnormal behavior, trouble walking, and weight loss. Later in the course of th ...
(mad cow disease),
chronic wasting disease
Chronic wasting disease (CWD), sometimes called zombie deer disease, is a transmissible spongiform encephalopathy (TSE) affecting deer. TSEs are a family of diseases thought to be caused by misfolded proteins called prions and include simila ...
,
scrapie
Scrapie () is a fatal, degenerative disease affecting the nervous systems of sheep and goats. It is one of several transmissible spongiform encephalopathies (TSEs), and as such it is thought to be caused by a prion. Scrapie has been known sin ...
,
transmissible mink encephalopathy,
feline spongiform encephalopathy, and ungulate spongiform encephalopathy.
Other animal diseases include a variety of immunodeficiency disorders caused by viruses related to human immunodeficiency virus (HIV), such as
BIV and
FIV.
Humans
Humans can be infected with many types of pathogens, including prions, viruses, bacteria, and fungi, causing symptoms like sneezing, coughing, fever, vomiting, and potentially lethal
organ failure
Organ dysfunction is a condition where an organ does not perform its expected function. Organ failure is organ dysfunction to such a degree that normal homeostasis cannot be maintained without external clinical intervention or life support. It i ...
. While some symptoms are caused by the pathogenic infection, others are caused by the immune system's efforts to kill the pathogen, such as feverishly high body temperatures meant to
denature pathogenic cells.
Treatment
Prions
Despite many attempts, no therapy has been shown to halt the progression of
prion diseases
Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are a group of progressive, incurable, and fatal conditions that are associated with the prion hypothesis and affect the brain and nervous system of many animals, in ...
.
Viruses
A variety of prevention and treatment options exist for some viral pathogens.
Vaccine
A vaccine is a biological Dosage form, preparation that provides active acquired immunity to a particular infectious disease, infectious or cancer, malignant disease. The safety and effectiveness of vaccines has been widely studied and verifi ...
s are one common and effective preventive measure against a variety of viral pathogens. Vaccines prime the immune system of the host, so that when the potential host encounters the virus in the wild, the immune system can defend against infection quickly. Vaccines designed against viruses include annual
influenza vaccine
Influenza vaccines, colloquially known as flu shots or the flu jab, are vaccines that protect against infection by influenza viruses. New versions of the vaccines are developed twice a year, as the influenza virus rapidly changes. While their ...
s and the two-dose
MMR vaccine
The MMR vaccine is a vaccine against measles, mumps, and rubella (German measles), abbreviated as ''MMR''. The first dose is generally given to children around 9 months to 15 months of age, with a second dose at 15 months to 6 years of age, w ...
against
measles
Measles (probably from Middle Dutch or Middle High German ''masel(e)'', meaning "blemish, blood blister") is a highly contagious, Vaccine-preventable diseases, vaccine-preventable infectious disease caused by Measles morbillivirus, measles v ...
,
mumps
MUMPS ("Massachusetts General Hospital Utility Multi-Programming System"), or M, is an imperative, high-level programming language with an integrated transaction processing key–value database. It was originally developed at Massachusetts Gen ...
, and
rubella
Rubella, also known as German measles or three-day measles, is an infection caused by the rubella virus. This disease is often mild, with half of people not realizing that they are infected. A rash may start around two weeks after exposure and ...
.
Vaccines are not available against the viruses responsible for
HIV/AIDS
The HIV, human immunodeficiency virus (HIV) is a retrovirus that attacks the immune system. Without treatment, it can lead to a spectrum of conditions including acquired immunodeficiency syndrome (AIDS). It is a Preventive healthcare, pr ...
,
dengue
Dengue fever is a mosquito-borne disease caused by dengue virus, prevalent in tropical and subtropical areas. Asymptomatic infections are uncommon, mild cases happen frequently; if symptoms appear, they typically begin 3 to 14 days after ...
, and
chikungunya
Chikungunya is an infection caused by the chikungunya virus (CHIKV). The disease was first identified in 1952 in Tanzania and named based on the Kimakonde words for "to become contorted". Chikungunya has become a global health concern due to ...
.
Treatment of viral infections often involves treating the symptoms of the infection, rather than providing medication to combat the viral pathogen itself. Treating the symptoms of a viral infection gives the host immune system time to develop antibodies against the viral pathogen. However, for HIV,
highly active antiretroviral therapy (HAART) is conducted to prevent the viral disease from progressing into AIDS as immune cells are lost.
Bacteria

Much like viral pathogens, infection by certain bacterial pathogens can be prevented via vaccines.
Vaccines against bacterial pathogens include the
anthrax vaccine
Anthrax vaccines are vaccines to prevent the livestock and human disease anthrax, caused by the bacterium ''Bacillus anthracis''.
They have had a prominent place in the history of medicine, from Pasteur's pioneering 19th-century work with cattle ...
and
pneumococcal vaccine
Pneumococcal vaccines are vaccines against the bacterium ''Streptococcus pneumoniae''. Their use can prevent some cases of pneumonia, meningitis, and sepsis. There are two types of pneumococcal vaccines: conjugate vaccines and polysaccharide ...
. Many other bacterial pathogens lack vaccines as a preventive measure, but infection by these bacteria can often be treated or prevented with
antibiotic
An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy ...
s. Common antibiotics include
amoxicillin
Amoxicillin is an antibiotic medication belonging to the aminopenicillin class of the penicillin family. The drug is used to treat bacterial infections such as middle ear infection, strep throat, pneumonia, skin infections, odontogenic inf ...
,
ciprofloxacin
Ciprofloxacin is a fluoroquinolone antibiotic used to treat a number of bacterial infections. This includes bone and joint infections, intra-abdominal infections, certain types of infectious diarrhea, respiratory tract infections, skin ...
, and
doxycycline
Doxycycline is a Broad-spectrum antibiotic, broad-spectrum antibiotic of the Tetracycline antibiotics, tetracycline class used in the treatment of infections caused by bacteria and certain parasites. It is used to treat pneumonia, bacterial p ...
. Each antibiotic has different bacteria that it is effective against and has different mechanisms to kill that bacteria. For example,
doxycycline
Doxycycline is a Broad-spectrum antibiotic, broad-spectrum antibiotic of the Tetracycline antibiotics, tetracycline class used in the treatment of infections caused by bacteria and certain parasites. It is used to treat pneumonia, bacterial p ...
inhibits the synthesis of new proteins in both
gram-negative
Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...
and
gram-positive bacteria
In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.
The Gram stain ...
, which makes it a
broad-spectrum antibiotic
A broad-antimicrobial spectrum, spectrum antibiotic is an antibiotic that acts on the two major bacterial groups, Gram-positive and Gram-negative, or any antibiotic that acts against a wide range of disease-causing bacteria. These medications are ...
capable of killing most bacterial species.
Due to misuse of antibiotics, such as prematurely ended prescriptions exposing bacteria to
evolutionary pressure
Evolutionary pressure, selective pressure or selection pressure is exerted by factors that reduce or increase reproductive success in a portion of a population, driving natural selection. It is a quantitative description of the amount of change o ...
under sublethal doses, some bacterial pathogens have developed
antibiotic resistance
Antimicrobial resistance (AMR or AR) occurs when microbes evolve mechanisms that protect them from antimicrobials, which are drugs used to treat infections. This resistance affects all classes of microbes, including bacteria (antibiotic resis ...
. For example, a genetically distinct strain of
Staphylococcus aureus
''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often posi ...
called
MRSA
Methicillin-resistant ''Staphylococcus aureus'' (MRSA) is a group of gram-positive bacteria that are genetically distinct from other strains of ''Staphylococcus aureus''. MRSA is responsible for several difficult-to-treat infections in humans. ...
is resistant to the commonly prescribed
beta-lactam antibiotics. A 2013 report from the
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 ...
(CDC) estimated that in the United States, at least 2 million people get an antibiotic-resistant bacterial infection annually, with at least 23,000 of those patients dying from the infection.
Due to their indispensability in combating bacteria, new antibiotics are required for medical care. One target for new antimicrobial medications involves inhibiting
DNA methyltransferase
In biochemistry, the DNA methyltransferase (DNA MTase, DNMT) family of enzymes catalyze the transfer of a methyl group to DNA. DNA methylation serves a wide variety of biological functions. All the known DNA methyltransferases use S-adenosyl ...
s, as these proteins control the levels of expression for other genes, such as those encoding virulence factors.
Fungi
Infection by fungal pathogens is treated with anti-fungal medication.
Athlete's foot
Athlete's foot, known medically as ''tinea pedis'', is a common skin infection of the feet caused by a fungus. Signs and symptoms often include itching, scaling, cracking and redness. In rare cases the skin may blister. Athlete's foot fungus ...
,
jock itch
Jock may refer to:
Common meanings
* Jock (stereotype), a North American term for a stereotypical male athlete
* Jock, a derogatory term for Scottish people mostly used by the English
* Short for jockstrap, an item of male protective undergarmen ...
, and
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 ...
are fungal skin infections that are treated with topical anti-fungal medications like
clotrimazole
Clotrimazole, sold under the brand name Lotrimin, among others, is an antifungal medication. It is used to treat vaginal yeast infections, oral thrush, diaper rash, tinea versicolor, and types of ringworm including athlete's foot and jock itc ...
. Infections involving the yeast species ''
Candida albicans
''Candida albicans'' is an opportunistic pathogenic yeast that is a common member of the human gut flora. It can also survive outside the human body. It is detected in the gastrointestinal tract and mouth in 40–60% of healthy adults. It is usu ...
'' cause
oral thrush and
vaginal yeast infection
Vaginal yeast infection, also known as candidal vulvovaginitis and vaginal thrush, is excessive growth of yeast in the vagina that results in irritation. The most common symptom is vaginal itching, which may be severe. Other symptoms include Dysur ...
s. These internal infections can either be treated with anti-fungal creams or with oral medication. Common anti-fungal drugs for internal infections include the
echinocandin
Echinocandins are a class of antifungal drugs that inhibit the synthesis of β-glucan in the fungal cell wall via noncompetitive inhibition of the enzyme 1,3-β glucan synthase. The class has been dubbed the "penicillin of antifungals," along w ...
family of drugs and
fluconazole
Fluconazole is an antifungal medication used for a number of fungal infections. These include candidiasis, blastomycosis, coccidioidomycosis, cryptococcosis, histoplasmosis, dermatophytosis, and tinea versicolor. It is also used to pr ...
.
Algae
While algae are commonly not thought of as pathogens, the genus ''
Prototheca
''Prototheca'' is a genus of algae in the family Chlorellaceae. While this genus is a member of the green algae, all ''Prototheca'' no longer have chloroplasts and therefore their photosynthetic ability. Some species can cause protothecosis in ...
'' causes disease in humans''.''
Treatment for
protothecosis
Protothecosis, otherwise known as Algaemia, is a disease found in dogs, cats, cattle, and humans caused by a type of green alga known as '' Prototheca'' that lacks chlorophyll and enters the human or animal bloodstream. It and its close relative ...
is currently under investigation, and there is no consistency in clinical treatment.
Sexual interactions
Many pathogens are capable of sexual interaction. Among
pathogenic bacteria
Pathogenic bacteria are bacteria that can cause disease. This article focuses on the bacteria that are pathogenic to humans. Most species of bacteria are harmless and many are Probiotic, beneficial but others can cause infectious diseases. The nu ...
, sexual interaction occurs between cells of the same species by the process of
genetic transformation
In molecular biology and genetics, transformation is the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous genetic material from its surroundings through the cell membrane(s). For transformation to t ...
. Transformation involves the transfer of
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 ...
from a donor cell to a recipient cell and the integration of the donor DNA into the recipient
genome
A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
through
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 bacterial pathogens ''
Helicobacter pylori
''Helicobacter pylori'', previously known as ''Campylobacter pylori'', is a gram-negative, Flagellum#bacterial, flagellated, Bacterial cellular morphologies#Helical, helical bacterium. Mutants can have a rod or curved rod shape that exhibits l ...
'', ''
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 ...
'', ''
Legionella pneumophila
''Legionella pneumophila'', the primary causative agent for Legionnaires' disease, Legionnaire's disease, is an Aerobic organism, aerobic, pleomorphic, Flagellum, flagellated, non-spore-forming, Gram-negative bacteria, Gram-negative bacterium. ' ...
'', ''
Neisseria gonorrhoeae
''Neisseria gonorrhoeae'', also known as ''gonococcus'' (singular) or ''gonococci'' (plural), is a species of Gram-negative diplococci bacteria first isolated by Albert Ludwig Sigesmund Neisser, Albert Neisser in 1879. An obligate human pathog ...
'', and ''
Streptococcus pneumoniae
''Streptococcus pneumoniae'', or pneumococcus, is a Gram-positive, spherical bacteria, hemolysis (microbiology), alpha-hemolytic member of the genus ''Streptococcus''. ''S. pneumoniae'' cells are usually found in pairs (diplococci) and do not f ...
'' frequently undergo transformation to modify their genome for additional traits and evasion of host immune cells.
Eukaryotic
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 ...
pathogens are often capable of sexual interaction by a process involving
meiosis
Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...
and
fertilization
Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...
. Meiosis involves the intimate pairing of homologous chromosomes and recombination between them. Examples of eukaryotic pathogens capable of sex include the
protozoan parasites ''
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 ...
'', ''
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 ...
'', ''
Trypanosoma brucei
''Trypanosoma brucei'' is a species of parasitic Kinetoplastida, kinetoplastid belonging to the genus ''Trypanosoma'' that is present in sub-Saharan Africa. Unlike other protozoan parasites that normally infect blood and tissue cells, it is excl ...
'', ''
Giardia intestinalis
''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellated parasitic protozoan microorganism of the genus ''Giardia'' that colonizes the small intestine, causing a diarrheal condition known as giard ...
'', and the fungi ''
Aspergillus fumigatus
''Aspergillus fumigatus'' is a species of fungus in the genus ''Aspergillus'', and is one of the most common ''Aspergillus'' species to cause disease in individuals with an immunodeficiency.
''Aspergillus fumigatus'', a saprotroph widespread in ...
'', ''
Candida albicans
''Candida albicans'' is an opportunistic pathogenic yeast that is a common member of the human gut flora. It can also survive outside the human body. It is detected in the gastrointestinal tract and mouth in 40–60% of healthy adults. It is usu ...
'' and ''
Cryptococcus neoformans
''Cryptococcus neoformans'' is an encapsulated basidiomycetous yeast belonging to the class Tremellomycetes and an obligate aerobe that can live in both plants and animals. Its teleomorph is a filamentous fungus, formerly referred to ''Filob ...
''.
Viruses may also undergo sexual interaction when two or more viral
genome
A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
s enter the same host cell. This process involves pairing of homologous genomes and recombination between them by a process referred to as multiplicity reactivation. The
herpes simplex virus
Herpes simplex virus 1 and 2 (HSV-1 and HSV-2) are two members of the Herpesviridae#Human herpesvirus types, human ''Herpesviridae'' family, a set of viruses that produce Viral disease, viral infections in the majority of humans. Both HSV-1 a ...
,
human immunodeficiency virus
The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause AIDS, acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of th ...
, and
vaccinia
The vaccinia virus (VACV or VV) is a large, complex, enveloped virus belonging to the poxvirus family. It has a linear, double-stranded DNA genome approximately 190 kbp in length, which encodes approximately 250 genes. The dimensions of the ...
virus undergo this form of sexual interaction.
These processes of sexual recombination between homologous genomes supports
repairs
The technical meaning of maintenance involves functional checks, servicing, repairing or replacing of necessary devices, equipment, machinery, building infrastructure and supporting utilities in industrial, business, and residential installa ...
to genetic damage caused by environmental stressors and host immune systems.
See also
*
Antigenic escape
*
Ecological competence
*
Emerging Pathogens Institute
*
Human pathogen
A human pathogen is a pathogen (microbe or microorganism such as a virus, bacterium, prion, or fungus) that causes disease in humans.
The human physiological defense against common pathogens (such as ''Pneumocystis'') is mainly the responsibility ...
*
Pathogen-Host Interaction Database (PHI-base)
References
External links
Pronunciation Guide to Microorganisms (1)Pronunciation Guide to Microorganisms (2)
{{Authority control
Infectious diseases
Microbiology
Hazardous materials