<pedantry> bus is a contraction of omnibus, which is the dative/ablative plural of omnis (all). -ibus is the normal ending for all III declension nouns and adjectives.

Virus looks like a II declension masculine noun, and the plural would be viri (virii would be the plural of virius). But if, as Deda says, it is a neuter masquerading as a masculine, then vira would be correct.

Probably no connection to vir (pl. vires), which means man </pedantry>

WassaiL! V