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

virus
computer virus
viruses

    (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)