Derived from the Latin virus (meaning roughly "poison") the word was used to describe biological viruses when these were first discovered to be infectious disease-causing agents smaller than bacteria that reproduce with the help of a host cell. It is also used metaphorically to refer to other parasitically-reproducing things like ideas. The term computer virus, once metaphorical, is now another well-defined sense of the word.
Despite frequent claims to the contrary, the only correct English plural of the word used in any of these senses is viruses. See, for example, .