I occasionally see people use "virii" as the plural of "virus," but there's no good reason (that I know of) to do so.

See Wikipedia's entry on the plural of virus for details. Excerpts:

The plural of virus is viruses. In reference to a computer virus, the plural is often believed to be virii or, less commonly, viri, but both forms are neologistic folk etymology and no major dictionary recognizes them as alternative forms.


Virus comes to English from Latin. The Latin word vīrus [...] means "poison; venom", denoting the venom of a snake. [...] Since vīrus in antiquity denoted something uncountable, it was a mass noun. Mass nouns—such as air, rice, and helpfulness in English—pluralize only under special circumstances, hence the non-existence of plural forms [of vīrus] in the texts.