Virus

A virus is a microscopic organism that lacks the basic cellular machinery to survive on its own. It must live inside the cells of a host organism, usually a person, animal or plant. Viruses multiple rapidly inside cells, and can cause disease by disrupting normal cellular functions. Whereas bacteria are alive, viruses exist in a gray area.

A computer virus also acts in much the same way, and is used to destroy, damage or manipulate a computer, web site or system.