Etymology
The word comes from the Latin virus, referring to poison and other noxious things. Today it is used to describe the biological viruses discussed above and also as a metaphor for other parasitically-reproducing things, such as ideas. The term computer virus has become another well-defined sense of the word. The word virion is used to refer to a single infective viral particle.
Despite frequent claims to the contrary, the only correct English plural of the word for any of these senses is viruses. The Latin word does not appear to have had a plural. Virii would be the plural of the word virius, and viri was the plural of the word vir, meaning man.