Formation of the plural of "virus" is treated in the article plural of virus. See also viral for metaphorical application of the virus concept (e.g. memes).

• In biology, a virus is a parasitic agent that is smaller than a bacterium and that can only reproduce after infecting a host cell.
• In computer science, a virus is a (usually malicious) computer program that can travel surreptitiously from computer to computer.
• 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)
• Virus is also a computer game by David Braben.