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".
Comments
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".
Neuter nouns of the second declension don't generally have plurals that end in -i, but rather in -a, so "vira" would be equally possible.
It's also important to note that scholars don't actually know the proper plural of virus because they haven't really found one in extant literature.
Wikipedia has a longer discussion at http://en.wikipedia.org/wiki/Plural_of_virus#Virus
Doo doo doo doo, doo doodoo do, Doo doo doo doo, doo doodoo do, Doo doo doo doo, doo doodoo do, Doo, doodoodoo, doo doo doo doo doo....
A bit hard to communicate, but that's the keyboard cat playing all of you off.