Being a Latin geek myself I can't help but point out that nouns in the fourth declension (u stem) also end in -us in singular and receive an -us affix in plural as well.

"Virus" is however, in the second declension (virus -i n. "slime, poison, goo") with the oddity of being neutral while having a second declension -us ending which is normally a feature of masculine nouns. And indeed, its plural would be "viri".