The Free On-line Dictionary of Computing (30 December 2018):
(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