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an ultramicroscopic (20 to 300 nm in diameter), metabolically inert, infectious agent that replicates only within the cells of living hosts, mainly bacteria, plants, and animals: composed of an RNA or DNA core, a protein coat, and, in more complex types, a surrounding envelope.
Informal. a disease.
a corrupting influence on morals or the intellect; poison.
a segment of self-replicating code planted illegally in a computer program, often to damage or shut down a system or network.
Contemporary Examples

With Athens recently alarmed by a half-dozen cases of West Nile virus, the attempt at humor went mostly unappreciated.
The Good, the Bad, and the Ugly of Being Politically Correct Michael Medved July 30, 2012

It also may relate to our still primitive understanding of the natural history of Ebola virus infection.
Emory Will Wage High-Tech War on Ebola Kent Sepkowitz July 31, 2014

The serum failed to neutralize the virus in subsequent tests and seemed to offer little protection in animal experiments.
Infected Ebola Doctor Kent Brantly Is an Endangered Hero Michael Daly August 2, 2014

A virus called PEDv is killing 100,000 pigs and piglets each week, and shows no signs of stopping.
Aporkalypse Now: Pig-Killing Virus Could Mean the End of Bacon Carrie Arnold August 19, 2014

He became delirious, his heartbeat grew ragged, his blood teemed with the virus, and his lungs, liver and kidneys began to fail.
The Daily Beast’s Best Longreads, Dec 8-14, 2014 William Boot December 12, 2014

Historical Examples

Such an idea is as fatal to society as we know it as a virus plague.
Suite Mentale Gordon Randall Garrett

They knew that shortly after every Nansalian died, the virus, too, would be dead.
Islands of Space John W Campbell

The exact nature of the virus is unknown, but it is probably bacterial.
The Fijians Basil Thomson

But what am I saying, A W, to you who are so free from the virus of culture?
Greener Than You Think Ward Moore

The original is in Latin: Velut e cœlo dejectus serpens, virus effundit in terras.
History of the Reformation of the Sixteenth Century, Volume III J. H. Merle D’Aubign

noun (pl) -ruses
any of a group of submicroscopic entities consisting of a single nucleic acid chain surrounded by a protein coat and capable of replication only within the cells of living organisms: many are pathogenic
(informal) a disease caused by a virus
any corrupting or infecting influence
(computing) an unauthorized program that inserts itself into a computer system and then propagates itself to other computers via networks or disks; when activated it interferes with the operation of the computer
n.

late 14c., “venomous substance,” from Latin virus “poison, sap of plants, slimy liquid,” probably from PIE root *weis- “to melt away, to flow,” used of foul or malodorous fluids, with specialization in some languages to “poisonous fluid” (cf. Sanskrit visam “poison,” visah “poisonous;” Avestan vish- “poison;” Latin viscum “sticky substance, birdlime;” Greek ios “poison,” ixos “mistletoe, birdlime; Old Church Slavonic višnja “cherry;” Old Irish fi “poison;” Welsh gwy “fluid, water,” gwyar “blood”). Main modern meaning “agent that causes infectious disease” first recorded 1728. The computer sense is from 1972.

virus vi·rus (vī’rəs)
n. pl. vi·rus·es

Any of various simple submicroscopic parasites of plants, animals, and bacteria that often cause disease and that consist essentially of a core of RNA or DNA surrounded by a protein coat. Unable to replicate without a host cell, viruses are typically not considered living organisms.

A disease caused by a virus.

virus
(vī’rəs)
Plural viruses

Any of various extremely small, often disease-causing agents consisting of a particle (the virion), containing a segment of RNA or DNA within a protein coat known as a capsid. Viruses are not technically considered living organisms because they are devoid of biological processes (such as metabolism and respiration) and cannot reproduce on their own but require a living cell (of a plant, animal, or bacterium) to make more viruses. Viruses reproduce first either by injecting their genetic material into the host cell or by fully entering the cell and shedding their protein coat. The genetic material may then be incorporated into the cell’s own genome or remain in the cytoplasm. Eventually the viral genes instruct the cell to produce new viruses, which often cause the cell to die upon their exit. Rather than being primordial forms of life, viruses probably evolved from rogue pieces of cellular nucleic acids. The common cold, influenza, chickenpox, smallpox, measles, mumps, yellow fever, hemorrhagic fevers, and some cancers are among the diseases caused by viruses.

Computer Science A computer program that duplicates itself in a manner that is harmful to normal computer use. Most viruses work by attaching themselves to another program. The amount of damage varies; viruses may erase all data or do nothing but reproduce themselves.

viral adjective
virus [(veye-ruhs)]

plur. viruses

Microorganisms consisting of DNA and RNA molecules wrapped in a protective coating of proteins. Viruses are the most primitive form of life. They depend on other living cells for their reproduction and growth. (See under “Medicine and Health.”)

Note: Viruses cause many diseases. (See viral infection.)

virus [(veye-ruhs)]

plur. viruses

A minute organism that consists of a core of nucleic acid surrounded by protein. Viruses, which are so small that a special kind of microscope is needed to view them, can grow and reproduce only inside living cells. (See under “Life Sciences.”)

See computer virus.
security
(By analogy with biological viruses, via science fiction) A program or piece of code, a type of malware, written by a cracker, that “infects” one or more other programs by embedding a copy of itself in them, so that they become Trojan horses. When these programs are executed, the embedded virus is executed too, thus propagating the “infection”. This normally happens invisibly to the user.
A virus has an “engine” – code that enables it to propagate and optionally a “payload” – what it does apart from propagating. It needs a “host” – the particular hardware and software environment on which it can run and a “trigger” – the event that starts it running.
Unlike a worm, a virus cannot infect other computers without assistance. It is propagated by vectors such as humans trading programs with their friends (see SEX). The virus may do nothing but propagate itself and then allow the program to run normally. Usually, however, after propagating silently for a while, it starts doing things like writing “cute” messages on the terminal or playing strange tricks with the display (some viruses include display hacks). Viruses written by particularly antisocial crackers may do irreversible damage, like deleting files.
By the 1990s, viruses had become a serious problem, especially among IBM PC and Macintosh users (the lack of security on these machines enables viruses to spread easily, even infecting the operating system). The production of special antivirus software has become an industry, and a number of exaggerated media reports have caused outbreaks of near hysteria among users. Many lusers tend to blame *everything* that doesn’t work as they had expected on virus attacks. Accordingly, this sense of “virus” has passed into popular usage where it is often incorrectly used for other types of malware such as worms or Trojan horses.
See boot virus, phage. Compare back door. See also Unix conspiracy.
[Jargon File]
(2003-06-20)