virii

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.