The word virus is derived from the Latin virus, meaning roughly "poison" or "venom", and still retains this meaning.

To see how virus is pluralized, see virus (plural).

• In biology, virus refers to parasitic agents that are smaller than bacteria and that can only reproduce after infecting a host cell; see virus (biology).
• In computer science, a virus is a (usually malicious) computer program that can travel surreptitiously from computer to computer; see computer virus.
• Virus is also used metaphorically to refer to other entities, such as ideas, that can spread rapidly by "infecting" their hosts (see also meme). Certain types of software licence have been called "viral" because they require that derivative works be released under the same licence, GPL being a prominent example.
• Virus is the title of no fewer than 5 feature films:
  • Virus (2002 movie)
  • Virus (2001 movie)
  • Virus (1999 movie)
  • Virus (1995 movie)
  • Virus (1981 movie)