Virus
Derived from the Latin virus (which referred to several unpleasant things like snake venom and pond slime) the word was first 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.