[from the obvious analogy with biological viruses, via SF]
n. A cracker program that searches out other programs and 'infects' them 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. 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 your display (some viruses include nice display hacks). Many nasty viruses, written by particularly perversely minded crackers, do irreversible damage, like nuking all the user's files.
In the 1990s, viruses have 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 anti-virus 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 not only into techspeak but into also popular usage (where it is often incorrectly used to denote a worm or even a Trojan horse).
Compare back door; see also UNIX conspiracy.