Virulent, from Latin virulentus ('poisonous'), dates to c. 1400. 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. See also: Plural form of words ending in -us Several classes of antiviral drugs have been developed. Some viruses, including those that cause HIV/AIDS, HPV infection, and viral hepatitis, evade these immune responses and result in chronic infections. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. Viral infections in animals provoke an immune response that usually eliminates the infecting virus.

This can be narrow, meaning a virus is capable of infecting few species, or broad, meaning it is capable of infecting many. The variety of host cells that a virus can infect is called its host range. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The infectious dose of norovirus required to produce infection in humans is fewer than 100 particles. 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. Many viruses, including influenza viruses, SARS-CoV-2, chickenpox, smallpox, and measles, spread in the air by coughing and sneezing. 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. Because they possess some but not all such qualities, viruses have been described as 'organisms at the edge of life' and as replicators. 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. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction. 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. Most virus species have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. 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. When infected, a host cell is often forced to rapidly produce thousands of copies of the original virus.

The study of viruses is known as virology, a subspeciality of microbiology. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. 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 infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. SARS-CoV-2, a member of the subfamily CoronavirinaeĪ virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. For a more accessible and less technical introduction to this topic, see Introduction to viruses.

Cursorcerer virus