virus virus a virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. since dmitri ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants and the discovery of the tobacco mosaic virus by martinus beijerinck in 1898, more than 11,000 of the millions of virus species have been described in detail. viruses are found in almost every ecosystem on earth and are the most numerous type of biological entity. the study of viruses is known as virology, a subspeciality of microbiology. virus sars-cov-2, a member of the subfamily coronavirinae virus classification (unranked): virus realms adnaviria duplodnaviria monodnaviria riboviria ribozyviria varidnaviria when infected, a host cell is often forced to rapidly produce thousands of copies of the original virus. when not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent viral particles, or virions, consisting of (i) the genetic material, i.e., long molecules of dna or rna that encode the structure of the proteins by which the virus acts; (ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids. the shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. most virus species have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria. the origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of dna that can move between cells—while others may have evolved from bacteria. in evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction. viruses are considered by some biologists to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack the key characteristics, such as cell structure, that are generally considered necessary criteria for defining life. because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life" and as replicators. viruses spread in many ways. one transmission pathway is through disease-bearing organisms known as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; and viruses in animals can be carried by blood-sucking insects. many viruses, including influenza viruses, sars-cov-2, chickenpox, smallpox, and measles, spread in the air by coughing and sneezing. norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by hand-to-mouth contact or in food or water. the infectious dose of norovirus required to produce infection in humans is fewer than 100 particles. hiv is one of several viruses transmitted through sexual contact and by exposure to infected blood. the variety of host cells that a virus can infect is called its host range. this can be narrow, meaning a virus is capable of infecting few species, or broad, meaning it is capable of infecting many. viral infections in animals provoke an immune response that usually eliminates the infecting virus. immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. some viruses, including those that cause hiv/aids, hpv infection, and viral hepatitis, evade these immune responses and result in chronic infections. several classes of antiviral drugs have been developed. etymology the word is from the latin neuter vīrus referring to poison and other noxious liquids, from the same indo-european base as sanskrit viṣa, avestan vīša, and ancient greek ἰός (all meaning 'poison'), first attested in english in 1398 in john trevisa's translation of bartholomeus anglicus's de proprietatibus rerum. virulent, from latin virulentus ('poisonous'), dates to c. 1400. a meaning of 'agent that causes infectious disease' is first recorded in 1728, long before the discovery of viruses by dmitri ivanovsky in 1892. the english plural is viruses (sometimes also vira), whereas the latin word is a mass noun, which has no classically attested plural (vīra is used in neo-latin). the adjective viral dates to 1948. the term virion (plural virions), which dates from 1959, is also used to refer to a single viral particle that is released from the cell and is capable of infecting other cells of the same type. origins viruses are found wherever there is life and have probably existed since living cells first evolved. the origin of viruses is unclear because they do not form fossils, so molecular techniques are used to investigate how they arose. in addition, viral genetic material occasionally integrates into the germline of the host organisms, by which they can be passed on vertically to the offspring of the host for many generations. this provides an invaluable source of information for paleovirologists to trace back ancient viruses that have existed up to millions of years ago. there are three main hypotheses that aim to explain the origins of viruses: regressive hypothesisviruses may have once been small cells that parasitised larger cells. over time, genes not required by their parasitism were lost. the bacteria rickettsia and chlamydia are living cells that, like viruses, can reproduce only inside host cells. they lend support to this hypothesis, as their dependence on parasitism is likely to have caused the loss of genes that enabled them to survive outside a cell. this is also called the 'degeneracy hypothesis', or 'reduction hypothesis'. cellular origin hypothesissome 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 plasmids (pieces of naked dna that can move between cells) or transposons (molecules of dna that replicate and move around to different positions within the genes of the cell). once called jumping genes, transposons are examples of mobile genetic elements and could be the origin of some viruses. they were discovered in maize by barbara mcclintock in 1950. this is sometimes called the 'vagrancy hypothesis', or the 'escape hypothesis'. co-evolution hypothesisthis is also called the 'virus-first hypothesis' and proposes that viruses may have evolved from complex molecules of protein and nucleic acid at the same time that cells first appeared on earth and would have been dependent on cellular life for billions of years. viroids are molecules of rna that are not classified as viruses because they lack a protein coat. they have characteristics that are common to several viruses and are often called subviral agents. viroids are important pathogens of plants. they do not code for proteins but interact with the host cell and use the host machinery for their replication. the hepatitis delta virus of humans has an rna genome similar to viroids but has a protein coat derived from hepatitis b virus and cannot produce one of its own. it is, therefore, a defective virus. although hepatitis delta virus genome may replicate independently once inside a host cell, it requires the help of hepatitis b virus to provide a protein coat so that it can be transmitted to new cells. in similar manner, the sputnik virophage is dependent on mimivirus, which infects the protozoan acanthamoeba castellanii. these viruses, which are dependent on the presence of other virus species in the host cell, are called 'satellites' and may represent evolutionary intermediates of viroids and viruses. in the past, there were problems with all of these hypotheses: the regressive hypothesis did not explain why even the smallest of cellular parasites do not resemble viruses in any way. the escape hypothesis did not explain the complex capsids and other structures on virus particles. the virus-first hypothesis contravened the definition of viruses in that they require host cells. viruses are now recognised as ancient and as having 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. the evidence for an ancestral world of rna cells and computer analysis of viral and host dna sequences are giving a better understanding of the evolutionary relationships between different viruses and may help identify the ancestors of modern viruses. to date, such analyses have not proved which of these hypotheses is correct. it seems unlikely that all currently known viruses have a common ancestor, and viruses have probably arisen numerous times in the past by one or more mechanisms. microbiology life properties scientific opinions differ on whether viruses are a form of life or organic structures that interact with living organisms. they have been described as "organisms at the edge of life", since they resemble organisms in that they possess genes, evolve by natural selection, and reproduce by creating multiple copies of themselves through self-assembly. although they have genes, they do not have a cellular structure, which is often seen as the basic unit of life. viruses do not have their own metabolism and require a host cell to make new products. they therefore cannot naturally reproduce outside a host cell—although some bacteria such as rickettsia and chlamydia are considered living organisms despite the same limitation. accepted forms of life use cell division to reproduce, whereas viruses spontaneously assemble within cells. they differ from autonomous growth of crystals as they inherit genetic mutations while being subject to natural selection. virus self-assembly within host cells has implications for the study of the origin of life, as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules. structure virions of some of the most common human viruses with their relative size. the nucleic acids are not to scale.diagram of how a virus capsid can be constructed using multiple copies of just two protein moleculesstructure of tobacco mosaic virus: rna coiled in a helix of repeating protein sub-unitsstructure of icosahedral adenovirus. electron micrograph with an illustration to show shapestructure of chickenpox virus. they have a lipid envelopestructure of an icosahedral cowpea mosaic virusbacteriophage escherichia virus ms2 capsid. this spherical virus also has icosahedral symmetry. viruses display a wide diversity of sizes and shapes, called 'morphologies'. in general, viruses are much smaller than bacteria and more than a thousand bacteriophage viruses would fit inside an escherichia coli bacterium's cell. many viruses that have been studied are spherical and have a diameter between 20 and 300 nanometres. some filoviruses, which are filaments, have a total length of up to 1400 nm; their diameters are only about 80 nm. most viruses cannot be seen with an optical microscope, so scanning and transmission electron microscopes are used to visualise them. to increase the contrast between viruses and the background, electron-dense "stains" are used. these are solutions of salts of heavy metals, such as tungsten, that scatter the electrons from regions covered with the stain. when virions are coated with stain (positive staining), fine detail is obscured. negative staining overcomes this problem by staining the background only. a complete virus particle, known as a virion, consists of nucleic acid surrounded by a protective coat of protein called a capsid. these are formed from protein subunits called capsomeres. viruses can have a lipid "envelope" derived from the host cell membrane. the capsid is made from proteins encoded by the viral genome and its shape serves as the basis for morphological distinction. virally-coded protein subunits will self-assemble to form a capsid, in general requiring the presence of the virus genome. complex viruses code for proteins that assist in the construction of their capsid. proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. the capsid and entire virus structure can be mechanically (physically) probed through atomic force microscopy. in general, there are five main morphological virus types: helicalthese viruses are composed of a single type of capsomere stacked around a central axis to form a helical structure, which may have a central cavity, or tube. this arrangement results in virions which can be short and highly rigid rods, or long and very flexible filaments. the genetic material (typically single-stranded rna, but single-stranded dna in some cases) is bound into the protein helix by interactions between the negatively charged nucleic acid and positive charges on the protein. overall, the