n.1.(Med.) Contagious or poisonous matter, as of specific ulcers, the bite of snakes, etc.; - applied to organic poisons.
2.the causative agent of a disease, .
3.any of numerous submicroscopic complex organic objects which have genetic material and may be considered as living organisms but have no proper cell membrane, and thus cannot by themselves perform metabolic processes, requiring entry into a host cell in order to multiply. The simplest viruses have no lipid envelope and may be considered as complex aggregates of molecules, sometimes only a nucleic acid (DNA or RNA) and a coat protein. They are sometimes viewed as being on the borderline between living and nonliving objects. They are smaller than living cells in size, usually between 20 and 300 nm; thus they pass through standard filters, and were previously referred to as filterable virus. The manifestations of disease caused by multiplication of viruses in cells may be due to destruction of the cells caused by subversion of the cellular metabolic processes by the virus, or by synthesis of a virus-specific toxin. Viruses may infect animals, plants, or microorganisms; those infecting bacteria are also called bacteriophages. Certain bacteriophages may be non-destructive and benign in the host; - see bacteriophage.
4.Fig.: Any morbid corrupting quality in intellectual or moral conditions; something that poisons the mind or the soul; as, the virus of obscene books.
5.(Computers) a program or segment of program code that may make copies of itself (replicate), attach itself to other programs, and perform unwanted actions within a computer; also called computer virus or virus program. Such programs are almost always introduced into a computer without the knowledge or assent of its owner, and are often malicious, causing destructive actions such as erasing data on disk, but sometime only annoying, causing peculiar objects to appear on the display. The form of sociopathic mental disease that causes a programmer to write such a program has not yet been given a name. Compare trojan horse{3}.
Noun1.virus - (virology) ultramicroscopic infectious agent that replicates itself only within cells of living hosts; many are pathogenic; a piece of nucleic acid (DNA or RNA) wrapped in a thin coat of protein
2.virus - a harmful or corrupting agency; "bigotry is a virus that must not be allowed to spread"; "the virus of jealousy is latent in everyone"
3.virus - a software program capable of reproducing itself and usually capable of causing great harm to files or other programs on the same computer; "a true virus cannot spread to another computer without human assistance"
Synonyms: computer virus
(security)virus - (By analogy with biological viruses, via SF) A program or piece of code 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 a worm or Trojan horse.

See boot virus, phage. Compare back door. See also Unix conspiracy.
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