Introduction To Viruses
   HOME

TheInfoList



OR:

A virus is a tiny
infectious agent 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 ...
that reproduces inside the
cells 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 d ...
of living
hosts 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 * ...
. When infected, the host cell is forced to rapidly produce thousands of identical copies of the original virus. Unlike most living things, viruses do not have cells that divide; new viruses assemble in the infected host cell. But unlike simpler infectious agents like
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, they contain
genes In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
, which allow them to
mutate In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitosis ...
and evolve. Over 4,800 species of viruses have been described in detail out of the millions in the environment. Their origin is unclear: some may have
evolved Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
from
plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria and ...
s—pieces of DNA that can move between cells—while others may have evolved from
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 ...
. Viruses are made of either two or three parts. All include
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
s. These genes contain the encoded biological information of the virus and are built from either
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 ...
or
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 ...
. All viruses are also covered with 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 to protect the genes. Some viruses may also have an
envelope An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter (message), letter or Greeting card, card. Traditional envelopes are made from sheets of paper cut to one o ...
of fat-like substance that covers the protein coat, and makes them vulnerable to soap. A virus with this "viral envelope" uses it—along with specific
receptors Receptor may refer to: *Sensory receptor, in physiology, any neurite structure that, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds ...
—to enter a new host cell. Viruses vary in shape from the simple helical and
icosahedral In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrical tha ...
to more
complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...
structures. Viruses range in size from 20 to 300
nanometre 330px, Different lengths as in respect to the Molecule">molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm), or nanometer (American spelling), is a unit of length ...
s; it would take 33,000 to 500,000 of them, laid end to end, to stretch to . Viruses spread in many ways. Although many are very specific about which host species or tissue they attack, each
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), ...
of virus relies on a particular method to copy itself. Plant viruses are often spread from plant to plant by insects and other
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 ...
s, known as '' vectors''. Some viruses of humans and other animals are spread by exposure to infected bodily fluids. Viruses 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 ...
are spread through the air by droplets of moisture when people cough or sneeze. Viruses such as
norovirus Norovirus, also known as Norwalk virus and sometimes referred to as the winter vomiting disease, is the most common cause of gastroenteritis. Infection is characterized by non-bloody diarrhea, vomiting, and stomach pain. Fever or headaches may ...
are transmitted by the faecal–oral route, which involves the contamination of hands, food and water.
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 ...
is often spread by direct contact with infected children. The human immunodeficiency virus,
HIV The human immunodeficiency viruses (HIV) are two species of '' Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the im ...
, is transmitted by bodily fluids transferred during sex. Others, such as the
dengue virus Dengue virus (DENV) is the cause of dengue fever. It is a mosquito-borne, single positive-stranded RNA virus of the family ''Flaviviridae''; genus '' Flavivirus''. Four serotypes of the virus have been found, and a reported fifth has yet to be ...
, are spread by blood-sucking insects. Viruses, especially those made of RNA, can
mutate In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitosis ...
rapidly to give rise to new types. Hosts may have little
protection Protection is any measure taken to guard something against damage caused by outside forces. Protection can be provided to physical objects, including organisms, to systems, and to intangible things like civil and political rights. Although ...
against such new forms. Influenza virus, for example, changes often, so a new
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 ...
is needed each year. Major changes can cause
pandemic A pandemic ( ) is an epidemic of an infectious disease that has a sudden increase in cases and spreads across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. Widespread endemic (epi ...
s, as in the 2009 swine influenza that spread to most countries. Often, these mutations take place when the virus has first infected other animal hosts. Some examples of such "zoonotic" diseases include
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 ...
in bats, and influenza in pigs and birds, before those viruses were transferred to humans. Viral infections can cause disease in humans, animals and plants. In healthy humans and animals, infections are usually eliminated by 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 ...
, which can provide lifetime
immunity Immunity may refer to: Medicine * Immunity (medical), resistance of an organism to infection or disease * ''Immunity'' (journal), a scientific journal published by Cell Press Biology * Immune system Engineering * Radiofrequence immunity ...
to the host for that virus.
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, which work against bacteria, have no impact, but
antiviral drug Antiviral drugs are a class of medication used for treating viral infections. Most antivirals target specific viruses, while a broad-spectrum antiviral is effective against a wide range of viruses. Antiviral drugs are a class of antimicrobials ...
s can treat life-threatening infections. Those vaccines that produce lifelong immunity can prevent some infections.


Discovery

In 1884, French
microbiologist A microbiologist (from Greek ) is a scientist who studies microscopic life forms and processes. This includes study of the growth, interactions and characteristics of microscopic organisms such as bacteria, algae, fungi, and some types of par ...
Charles Chamberland Charles Edouard Chamberland (; 12 March 1851 – 2 May 1908) was a French microbiologist from Chilly-le-Vignoble in the department of Jura who worked with Louis Pasteur. Chamberland was present at Pouilly-le-Fort when the efficacy of the anthr ...
invented the
Chamberland filter A Chamberland filter, also known as a Pasteur–Chamberland filter, is a porcelain water filter invented by Charles Chamberland in 1884. It was developed after Henry Doulton's ceramic water filter of 1827. It is similar to the Berkefeld fil ...
(or Chamberland–Pasteur filter), that contains pores smaller than
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 ...
. He could then pass a solution containing bacteria through the filter, and completely remove them. In the early 1890s, Russian
biologist A biologist is a scientist who conducts research in biology. Biologists are interested in studying life on Earth, whether it is an individual Cell (biology), cell, a multicellular organism, or a Community (ecology), community of Biological inter ...
Dmitri Ivanovsky Dmitri Iosifovich Ivanovsky (alternative spelling ''Dmitrii'' or ''Dmitry Iwanowski''; ; 28 October 1864 – 20 June 1920) was a Russian botanist, the co-discoverer of :viruses (1892), and one of the founders of virology. Life Ivanovsky was bo ...
used this method to study what became known as 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 ...
. His experiments showed that extracts from the crushed leaves of infected tobacco plants remain infectious after filtration. At the same time, several other scientists showed that, although these agents (later called viruses) were different from bacteria and about one hundred times smaller, they could still cause disease. In 1899, Dutch microbiologist
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 ...
observed that the agent only multiplied when in dividing cells. He called it a "contagious living fluid" ()—or a "soluble living germ" because he could not find any germ-like particles. In the early 20th century, English
bacteriologist A bacteriologist is a microbiologist, or similarly trained professional, in bacteriology— a subdivision of microbiology that studies bacteria, typically Pathogenic bacteria, pathogenic ones. Bacteriologists are interested in studying and learnin ...
Frederick Twort Frederick William Twort FRS (22 October 1877 – 20 March 1950) was an English bacteriologist and was the original discoverer in 1915 of bacteriophages (viruses that infect bacteria). He studied medicine at St Thomas's Hospital, London, was sup ...
discovered viruses that infect bacteria, and French-Canadian microbiologist
Félix d'Herelle Felix may refer to: * Felix (name), people and fictional characters with the name Places * Arabia Felix is the ancient Latin name of Yemen * Felix, Spain, a municipality of the province Almería, in the autonomous community of Andalusia, ...
described viruses that, when added to bacteria growing on
agar Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from " ogonori" and " tengusa". As found in nature, agar is a mixture of two components, t ...
, would lead to the formation of whole areas of dead bacteria. Counting these dead areas allowed him to calculate the number of viruses in the suspension. The invention of the
electron microscope An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing it ...
in 1931 brought the first images of viruses. In 1935, American
biochemist Biochemists are scientists who are trained in biochemistry. They study chemical processes and chemical transformations in living organisms. Biochemists study DNA, proteins and Cell (biology), cell parts. The word "biochemist" is a portmanteau of ...
and
virologist Virology is the scientific study of biological viruses. It is a subfield of microbiology that focuses on their detection, structure, classification and evolution, their methods of infection and exploitation of host cells for reproduction, the ...
Wendell Meredith Stanley Wendell Meredith Stanley (August 16, 1904June 15, 1971) was an American biochemist, virologist and Nobel laureate. Biography Stanley was born in Ridgeville, Indiana, and earned a BSc in chemistry at Earlham College in Richmond, Indiana. He th ...
examined the tobacco mosaic virus (TMV) and found it to be mainly made from
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 ...
. A short time later, this virus was shown to be made from protein and
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 ...
.
Rosalind Franklin Rosalind Elsie Franklin (25 July 192016 April 1958) was a British chemist and X-ray crystallographer. Her work was central to the understanding of the molecular structures of DNA (deoxyribonucleic acid), RNA (ribonucleic acid), viruses, coal ...
developed X-ray crystallographic pictures and determined the full structure of TMV in 1955. Franklin confirmed that viral proteins formed a spiral hollow tube, wrapped by RNA, and also showed that viral RNA was a single strand, not a double helix like DNA. A problem for early scientists was that they did not know how to grow viruses without using live animals. The breakthrough came in 1931, when American
pathologist 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 ...
s
Ernest William Goodpasture Ernest William Goodpasture (October 17, 1886 – September 20, 1960) was an American pathologist and physician. Goodpasture advanced the scientific understanding of the pathogenesis of infectious diseases, parasitism, and a variety of rickettsia ...
and
Alice Miles Woodruff Alice Miles Woodruff (November 29, 1900 – November 24, 1985), born Alice Lincoln Miles, was an American virologist. She developed a method for growing fowlpox outside of a live chicken alongside Ernest William Goodpasture. Her research grea ...
grew
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 several other viruses, in fertilised chickens' eggs. Some viruses could not be grown in chickens' eggs. This problem was solved in 1949, when
John Franklin Enders John Franklin Enders (February 10, 1897 – September 8, 1985) was an American biomedical scientist and Nobel Laureate. Enders has been called "The Father of Modern Vaccines." Life and education Enders was born in West Hartford, Connecticut on Fe ...
,
Thomas Huckle Weller Thomas Huckle Weller (June 15, 1915 – August 23, 2008) was an American virologist. He, John Franklin Enders and Frederick Chapman Robbins were awarded a Nobel Prize in Physiology or Medicine in 1954 for showing how to cultivate poliomyelitis ...
, and
Frederick Chapman Robbins Frederick Chapman Robbins (August 25, 1916 – August 4, 2003) was an American pediatrician and virologist. He was born in Auburn, Alabama, and grew up in Columbia, Missouri, attending David H. Hickman High School. He received the Nobel Prize in ...
grew
polio virus Poliovirus, the causative agent of polio (also known as poliomyelitis), is a serotype of the species '' Enterovirus C'', in the family of '' Picornaviridae''. There are three poliovirus serotypes, numbered 1, 2, and 3. Poliovirus is composed ...
in cultures of living animal cells. Over 4,800 species of viruses have been described in detail.


Origins

Viruses co-exist with life wherever it occurs. They have probably existed since living cells first evolved. Their origin remains unclear because they do not
fossil 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 preserve ...
ize, so
molecular techniques Molecular biology is a branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including biomolecular synthesis, modification, mechanisms, and interactions. Though cells and other microscop ...
have been the best way to
hypothesis A hypothesis (: hypotheses) is a proposed explanation for a phenomenon. A scientific hypothesis must be based on observations and make a testable and reproducible prediction about reality, in a process beginning with an educated guess o ...
e about how they arose. These techniques rely on the availability of ancient viral DNA or RNA, but most viruses that have been preserved and stored in laboratories are less than 90 years old. Molecular methods have only been successful in tracing the ancestry of viruses that evolved in the 20th century. New groups of viruses might have repeatedly emerged at all stages of the evolution of life. There are three major
theories A theory is a systematic and rational form of abstract thinking about a phenomenon, or the conclusions derived from such thinking. It involves contemplative and logical reasoning, often supported by processes such as observation, experimentation, ...
about the origins of viruses: ; Regressive theory : Viruses may have once been small cells that parasitised larger cells. Eventually, the genes they no longer needed for a parasitic way of life were lost. The bacteria ''
Rickettsia ''Rickettsia'' is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The genus was n ...
'' and ''
Chlamydia Chlamydia, or more specifically a chlamydia infection, is a sexually transmitted infection caused by the bacterium ''Chlamydia trachomatis''. Most people who are infected have no symptoms. When symptoms do appear, they may occur only several w ...
'' are living cells that, like viruses, can reproduce only inside host cells. This lends credence to this theory, as their dependence on being parasites may have led to the loss of the genes that once allowed them to live on their own. ; Cellular origin theory : Some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism. The escaped DNA could have come from
plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria and ...
s—pieces of DNA that can move between cells—while others may have evolved from bacteria. ; Coevolution theory : Viruses may have evolved from complex molecules of protein and DNA at the same time as cells first appeared on earth, and would have depended on cellular life for many millions of years. There are problems with all of these theories. The regressive hypothesis does not explain why even the smallest of cellular parasites do not resemble viruses in any way. The escape or the cellular origin hypothesis does not explain the presence of unique structures in viruses that do not appear in cells. The coevolution, or "virus-first" hypothesis, conflicts with the definition of viruses, because viruses depend on host cells. Also, viruses are recognised as ancient, and to have origins that pre-date the divergence of life into the three domains. This discovery has led modern virologists to reconsider and re-evaluate these three classical hypotheses.


Structure

A virus particle, also called a
virion A virion (plural, ''viria'' or ''virions'') is an inert virus particle capable of invading a Cell (biology), cell. Upon entering the cell, the virion disassembles and the genetic material from the virus takes control of the cell infrastructure, t ...
, consists of genes made from DNA or RNA which are surrounded by a protective coat of protein called a
capsid A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or m ...
. The capsid is made of many smaller, identical protein molecules called capsomers. The arrangement of the capsomers can either be
icosahedral In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrical tha ...
(20-faced), helical, or more complex. There is an inner shell around the DNA or RNA called the
nucleocapsid A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or ma ...
, made out of proteins. Some viruses are surrounded by a bubble of
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 ...
(fat) called an
envelope An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter (message), letter or Greeting card, card. Traditional envelopes are made from sheets of paper cut to one o ...
, which makes them vulnerable to soap and alcohol.


Size

Viruses are among the smallest infectious agents, and are too small to be seen by
light microscopy Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical, el ...
; most of them can only be seen by
electron microscopy An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing i ...
. Their sizes range from 20 to 300 
nanometre 330px, Different lengths as in respect to the Molecule">molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm), or nanometer (American spelling), is a unit of length ...
s; it would take 30,000 to 500,000 of them, laid end to end, to stretch to one centimetre (0.4 in). In comparison, bacteria are typically around 1000 nanometres (1 
micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * Micrometre The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights ...
) in diameter, and host cells of higher organisms are typically a few tens of micrometers. Some viruses, such as
megavirus ''Megavirus'' is a viral genus, phylogenetically related to '' Acanthamoeba polyphaga mimivirus'' (APMV). In colloquial speech, ''Megavirus chilense'' is more commonly referred to as just "Megavirus". Until the discovery of pandoraviruses in 2 ...
es and
pandoravirus ''Pandoravirus'' is a proposed genus of giant virus, first discovered in 2013. It is the third largest in physical size of any known viral genus, behind Pithovirus and Megaklothovirus. Pandoraviruses have double stranded DNA genomes, with t ...
es, are relatively large viruses. At around 1000 nanometres, these viruses, which infect
amoeba An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by ...
e, were discovered in 2003 and 2013. They are around ten times wider (and thus a thousand times larger in volume) than
influenza virus ''Orthomyxoviridae'' () is a family of negative-sense RNA viruses. It includes nine genera: '' Alphainfluenzavirus'', '' Betainfluenzavirus'', '' Gammainfluenzavirus'', '' Deltainfluenzavirus'', '' Isavirus'', '' Mykissvirus'', '' Quaranjavir ...
es, and the discovery of these "giant" viruses astonished scientists.


Genes

The genes of viruses are made from DNA (deoxyribonucleic acid) and, in many viruses, RNA (ribonucleic acid). The biological information contained in an organism is
encoded In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communication ...
in its DNA or RNA. Most organisms use DNA, but many viruses have RNA as their genetic material. The DNA or RNA of viruses consists of either a single strand or a double helix. Viruses can reproduce rapidly because they have relatively few genes. For example, influenza virus has only eight genes and
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 ...
has eleven. In comparison, humans have 20,000–25,000. Some viral genes contain the code to make the structural proteins that form the virus particle. Other genes make non-structural proteins found only in the cells the virus infects. All cells, and many viruses, produce proteins that are
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 that drive chemical reactions. Some of these enzymes, called
DNA polymerase A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create t ...
and
RNA polymerase In molecular biology, RNA polymerase (abbreviated RNAP or RNApol), or more specifically DNA-directed/dependent RNA polymerase (DdRP), is an enzyme that catalyzes the chemical reactions that synthesize RNA from a DNA template. Using the e ...
, make new copies of DNA and RNA. A virus's polymerase enzymes are often much more efficient at making DNA and RNA than the equivalent enzymes of the host cells, but viral RNA polymerase enzymes are error-prone, causing RNA viruses to mutate and form new strains. In some species of RNA virus, the genes are not on a continuous molecule of RNA, but are separated. The influenza virus, for example, has eight separate genes made of RNA. When two different strains of influenza virus infect the same cell, these genes can mix and produce new strains of the virus in a process called
reassortment Reassortment is the mixing of the genetic material of a species into new combinations in different individuals. The product of reassortment is called a reassortant. It is particularly used when two similar viruses that are infecting the same cell ...
.


Protein synthesis

Proteins are essential to life. Cells produce new protein molecules from
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
building blocks based on information coded in DNA. Each type of protein is a specialist that usually only performs one function, so if a cell needs to do something new, it must make a new protein. Viruses force the cell to make new proteins that the cell does not need, but are needed for the virus to reproduce.
Protein synthesis Protein biosynthesis, or protein synthesis, is a core biological process, occurring inside cells, balancing the loss of cellular proteins (via degradation or export) through the production of new proteins. Proteins perform a number of critica ...
consists of two major steps:
transcription Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, often th ...
and
translation Translation is the communication of the semantics, meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The English la ...
. Transcription is the process where information in DNA, called the
genetic code Genetic code is a set of rules used by living cell (biology), cells to Translation (biology), translate information encoded within genetic material (DNA or RNA sequences of nucleotide triplets or codons) into proteins. Translation is accomplished ...
, is used to produce RNA copies called
messenger RNA 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 synthesizing a protein. mRNA is created during the ...
(mRNA). These migrate through the cell and carry the code to
ribosome Ribosomes () are molecular machine, macromolecular machines, found within all cell (biology), cells, that perform Translation (biology), biological protein synthesis (messenger RNA translation). Ribosomes link amino acids together in the order s ...
s where it is used to make proteins. This is called translation because the protein's amino acid structure is determined by the mRNA's code. Information is hence translated from the language of nucleic acids to the language of amino acids. Some nucleic acids of RNA viruses function directly as mRNA without further modification. For this reason, these viruses are called positive-sense RNA viruses. In other RNA viruses, the RNA is a complementary copy of mRNA and these viruses rely on the cell's or their own enzyme to make mRNA. These are called
negative-sense In molecular biology and genetics, the sense of a nucleic acid molecule, particularly of a strand of DNA or RNA, refers to the nature of the roles of the strand and its complement in specifying a sequence of amino acids. Depending on the context, ...
RNA viruses. In viruses made from DNA, the method of mRNA production is similar to that of the cell. The species of viruses called
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 behave completely differently: they have RNA, but inside the host cell a DNA copy of their RNA is made with the help of the enzyme
reverse transcriptase A reverse transcriptase (RT) is an enzyme used to convert RNA genome to DNA, a process termed reverse transcription. Reverse transcriptases are used by viruses such as HIV and hepatitis B to replicate their genomes, by retrotransposon mobi ...
. This DNA is then incorporated into the host's own DNA, and copied into mRNA by the cell's normal pathways.


Life-cycle

When a virus infects a cell, the virus forces it to make thousands more viruses. It does this by making the cell copy the virus's DNA or RNA, making viral proteins, which all assemble to form new virus particles. There are six basic, overlapping stages in the life cycle of viruses in living cells: *Attachment is the binding of the virus to specific molecules on the surface of the cell. This specificity restricts the virus to a very limited type of cell. For example, the human immunodeficiency virus (HIV) infects only human
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, because its surface protein,
gp120 Envelope glycoprotein GP120 (or gp120) is a glycoprotein exposed on the surface of the HIV envelope. It was discovered by Professors Tun-Hou Lee and Myron "Max" Essex of the Harvard School of Public Health in 1984. The 120 in its name comes f ...
, can only react with
CD4 In molecular biology, CD4 (cluster of differentiation 4) is a glycoprotein that serves as a co-receptor for the T-cell receptor (TCR). CD4 is found on the surface of immune cells such as helper T cells, monocytes, macrophages, and dendritic c ...
and other molecules on the T cell's surface. Plant viruses can only attach to plant cells and cannot infect animals. This mechanism has evolved to favour those viruses that only infect cells in which they are capable of reproducing. *Penetration follows attachment; viruses penetrate the host cell by
endocytosis Endocytosis is a cellular process in which Chemical substance, substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a Vesicle (biology and chem ...
or by fusion with the cell. *Uncoating happens inside the cell when the viral capsid is removed and destroyed by viral enzymes or host enzymes, thereby exposing the viral nucleic acid. *Replication of virus particles is the stage where a cell uses viral messenger RNA in its protein synthesis systems to produce viral proteins. The RNA or DNA synthesis abilities of the cell produce the virus's DNA or RNA. *Assembly takes place in the cell when the newly created viral proteins and nucleic acid combine to form hundreds of new virus particles. *Release occurs when the new viruses escape or are released from the cell. Most viruses achieve this by making the cells burst, a process called
lysis Lysis ( ; from Greek 'loosening') is the breaking down of the membrane of a cell, often by viral, enzymic, or osmotic (that is, "lytic" ) mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a ...
. Other viruses such as HIV are released more gently by a process called
budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is kno ...
.


Effects on the host cell

Viruses have an extensive range of structural and biochemical effects on the host cell. These are called ''
cytopathic effect Cytopathic effect (abbreviated CPE) refers to structural changes in host cells that are caused by viral invasion. The infecting virus causes lysis of the host cell or when the cell dies without lysis due to an inability to replicate. If a virus c ...
s''. Most virus infections eventually result in the death of the host cell. The causes of death include cell lysis (bursting), alterations to the cell's surface membrane and
apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
(cell "suicide"). Often cell death is caused by cessation of its normal activity due to proteins produced by the virus, not all of which are components of the virus particle. Some viruses cause no apparent changes to the infected cell. Cells in which the virus is
latent Latency or latent may refer to: Engineering * Latency (engineering), a measure of the time delay experienced by a system ** Latency (audio), the delay between the moment an audio signal is triggered and the moment it is produced or received ** Mec ...
(inactive) show few signs of infection and often function normally. This causes persistent infections and the virus is often dormant for many months or years. This is often the case with herpes viruses. Some viruses, such as
Epstein–Barr virus The Epstein–Barr virus (EBV), also known as human herpesvirus 4 (HHV-4), is one of the nine known Herpesviridae#Human herpesvirus types, human herpesvirus types in the Herpesviridae, herpes family, and is one of the most common viruses in ...
, often cause cells to proliferate without causing
malignancy Malignancy () is the tendency of a medical condition to become progressively worse; the term is most familiar as a characterization of cancer. A ''malignant'' tumor contrasts with a non-cancerous ''benign'' tumor in that a malignancy is not ...
; but some other viruses, such as
papillomavirus ''Papillomaviridae'' is a family of non- enveloped double-stranded DNA viruses whose members are known as papillomaviruses. Several hundred species of papillomaviruses, traditionally referred to as "types", have been identified infecting all car ...
, are an established cause of cancer. When a cell's DNA is damaged by a virus such that the cell cannot repair itself, this often triggers apoptosis. One of the results of apoptosis is destruction of the damaged DNA by the cell itself. Some viruses have mechanisms to limit apoptosis so that the host cell does not die before progeny viruses have been produced;
HIV The human immunodeficiency viruses (HIV) are two species of '' Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the im ...
, for example, does this.


Viruses and diseases

There are many ways in which viruses spread from host to host but each species of virus uses only one or two. Many viruses that infect plants are carried by
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 ...
s; such organisms are called vectors. Some viruses that infect animals, including humans, are also spread by vectors, usually blood-sucking insects, but direct transmission is more common. Some virus infections, such as
norovirus Norovirus, also known as Norwalk virus and sometimes referred to as the winter vomiting disease, is the most common cause of gastroenteritis. Infection is characterized by non-bloody diarrhea, vomiting, and stomach pain. Fever or headaches may ...
and
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 ...
, are spread by contaminated food and water, by hands and communal
objects Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ai ...
, and by intimate contact with another infected person, while others like
SARS-CoV-2 Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a strain of coronavirus that causes COVID-19, the respiratory illness responsible for the COVID-19 pandemic. The virus previously had the Novel coronavirus, provisional nam ...
and influenza viruses are airborne. Viruses such as HIV,
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. ...
and
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 ...
are often transmitted by unprotected sex or contaminated
hypodermic needle A hypodermic needle (from Greek Language, Greek ὑπο- (''hypo-'' = under), and δέρμα (''derma'' = skin)) is a very thin, hollow tube with one sharp tip. As one of the most important intravenous inventions in the field of drug admini ...
s. To prevent infections and epidemics, it is important to know how each different kind of virus is spread.


In humans

Common human diseases caused by viruses include the
common cold The common cold, or the cold, is a virus, viral infectious disease of the upper respiratory tract that primarily affects the Respiratory epithelium, respiratory mucosa of the human nose, nose, throat, Paranasal sinuses, sinuses, and larynx. ...
,
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 ...
,
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
cold sores A cold sore is a type of herpes infection caused by the herpes simplex virus that affects primarily the lip. Symptoms typically include a burning pain followed by small blisters or sores. The first attack may also be accompanied by fever, so ...
. Serious diseases such as
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 ...
and
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 ...
are also caused by viruses. Many viruses cause little or no disease and are said to be "benign". The more harmful viruses are described as
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 ...
. Viruses cause different diseases depending on the types of cell that they infect. Some viruses can cause lifelong or chronic infections where the viruses continue to reproduce in the body despite the host's defence mechanisms. This is common in hepatitis B virus and hepatitis C virus infections. People chronically infected with a virus are known as carriers. They serve as important reservoirs of the virus.


Endemic

If the proportion of carriers in a given population reaches a given threshold, a disease is said to be
endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...
. Before the advent of vaccination, infections with viruses were common and outbreaks occurred regularly. In countries with a temperate climate, viral diseases are usually seasonal.
Poliomyelitis Poliomyelitis ( ), commonly shortened to polio, is an infectious disease caused by the poliovirus. Approximately 75% of cases are asymptomatic; mild symptoms which can occur include sore throat and fever; in a proportion of cases more severe ...
, caused by
poliovirus Poliovirus, the causative agent of polio (also known as poliomyelitis), is a serotype of the species '' Enterovirus C'', in the family of '' Picornaviridae''. There are three poliovirus serotypes, numbered 1, 2, and 3. Poliovirus is composed ...
often occurred in the summer months. By contrast colds, influenza and rotavirus infections are usually a problem during the winter months. Other viruses, such as
measles virus The measles virus (MV), with scientific name ''Morbillivirus hominis'', is a single-stranded, negative-sense, enveloped, non-segmented RNA virus of the genus ''Morbillivirus'' within the family ''Paramyxoviridae''. It is the cause of measles. H ...
, caused outbreaks regularly every third year. In developing countries, viruses that cause respiratory and enteric infections are common throughout the year. Viruses carried by insects are a common cause of diseases in these settings.
Zika Zika fever, also known as Zika virus disease or simply Zika, is an infectious disease caused by the Zika virus. Most cases have no symptoms, but when present they are usually mild and can resemble dengue fever. Symptoms may include fever, conju ...
and
dengue virus Dengue virus (DENV) is the cause of dengue fever. It is a mosquito-borne, single positive-stranded RNA virus of the family ''Flaviviridae''; genus '' Flavivirus''. Four serotypes of the virus have been found, and a reported fifth has yet to be ...
es for example are transmitted by female
Aedes ''Aedes'' (also known as the tiger mosquito) is a genus of mosquitoes originally found in tropical and subtropical zones, but now found on all continents except Antarctica. Some species have been spread by human activity: ''Aedes albopictus'', ...
mosquitoes, which bite humans particularly during the mosquitoes' breeding season.


Pandemic and emergent

Although viral
pandemic A pandemic ( ) is an epidemic of an infectious disease that has a sudden increase in cases and spreads across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. Widespread endemic (epi ...
s are rare events, HIV—which evolved from viruses found in monkeys and chimpanzees—has been pandemic since at least the 1980s. During the 20th century there were four pandemics caused by influenza virus and those that occurred in
1918 The ceasefire that effectively ended the World War I, First World War took place on the eleventh hour of the eleventh day of the eleventh month of this year. Also in this year, the Spanish flu pandemic killed 50–100 million people wor ...
,
1957 Events January * January 1 – The Saarland joins West Germany. * January 3 – Hamilton Watch Company introduces the first electric watch. * January 5 – South African player Russell Endean becomes the first batsman to be Dismissal (cricke ...
and
1968 Events January–February * January 1968, January – The I'm Backing Britain, I'm Backing Britain campaign starts spontaneously. * January 5 – Prague Spring: Alexander Dubček is chosen as leader of the Communist Party of Cze ...
were severe. Before its eradication, smallpox was a cause of pandemics for more than 3,000 years. Throughout history, human migration has aided the spread of pandemic infections; first by sea and in modern times also by air. With the exception of smallpox, most pandemics are caused by newly evolved viruses. These "emergent" viruses are usually mutants of less harmful viruses that have circulated previously either in humans or in other animals.
Severe acute respiratory syndrome Severe acute respiratory syndrome (SARS) is a viral respiratory disease of zoonotic origin caused by the virus SARS-CoV-1, the first identified strain of the SARS-related coronavirus. The first known cases occurred in November 2002, and the sy ...
(SARS) and
Middle East respiratory syndrome Middle East respiratory syndrome (MERS) is a viral respiratory infection caused by '' Middle East respiratory syndrome–related coronavirus'' (MERS-CoV). Symptoms may range from none, to mild, to severe depending on age and risk level. Typi ...
(MERS) are caused by new types of
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. Other coronaviruses are known to cause mild infections in humans, so the virulence and rapid spread of SARS infections—that by July 2003 had caused around 8,000 cases and 800 deaths—was unexpected and most countries were not prepared. A related coronavirus emerged in
Wuhan Wuhan; is the capital of Hubei, China. With a population of over eleven million, it is the most populous city in Hubei and the List of cities in China by population, eighth-most-populous city in China. It is also one of the nine National cent ...
, China, in November 2019 and spread rapidly around the world. Thought to have originated in bats and subsequently named
severe acute respiratory syndrome coronavirus 2 Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a strain of coronavirus that causes COVID-19, the respiratory illness responsible for the COVID-19 pandemic. The virus previously had the Novel coronavirus, provisional nam ...
, infections with the virus cause a disease called
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 ...
, that varies in severity from mild to deadly, and led to a pandemic in 2020. Restrictions unprecedented in peacetime were placed on international travel, and
curfews A curfew is an order that imposes certain regulations during specified hours. Typically, curfews order all people affected by them to remain indoors during the evening and nighttime hours. Such an order is most often issued by public authorit ...
imposed in several major cities worldwide.


In plants

There are many types of
plant virus Plant viruses are viruses that have the potential to affect plants. Like all other viruses, plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host. Plant viruses can be pathoge ...
, but often they only cause a decrease in yield, and it is not economically viable to try to control them. Plant viruses are frequently spread from plant to plant by organisms called " vectors". These are normally insects, but some
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 ...
,
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
single-celled organisms A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...
have also been shown to be vectors. When control of plant virus infections is considered economical (perennial fruits, for example) efforts are concentrated on killing the vectors and removing alternate hosts such as weeds. Plant viruses are harmless to humans and other animals because they can only reproduce in living plant cells.


Bacteriophages

Bacteriophages are viruses that infect 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 ...
. They are important in
marine ecology Marine ecosystems are the largest of Earth's aquatic ecosystems and exist in waters that have a high salt content. These systems contrast with freshwater ecosystems, which have a lower salt content. Marine waters cover more than 70% of the sur ...
: as the infected bacteria burst, carbon compounds are released back into the environment, which stimulates fresh organic growth. Bacteriophages are useful in scientific research because they are harmless to humans and can be studied easily. These viruses can be a problem in industries that produce food and drugs by
fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...
and depend on healthy bacteria. Some bacterial infections are becoming difficult to control with antibiotics, so there is a growing interest in
phage therapy Phage therapy, viral phage therapy, or phagotherapy is the therapeutic use of bacteriophages for the treatment of pathogenic bacterial infections. This therapeutic approach emerged at the beginning of the 20th century but was progressively r ...
, the use of bacteriophages to treat infections in humans.


Host resistance


Innate immunity of animals

Animals, including humans, have many natural defences against viruses. Some are non-specific and protect against many viruses regardless of the type. This
innate {{Short pages monitor