[thomaspaine]

Virus is Latin for poison. It's a mass noun because it denotes something uncountable (not in the strict mathematical sense, but in the how the hell do you count poison sense). As far as I know, there is no Latin plural form for virus.

Second declension singular nominative nouns end in 'us' and their plural form end in 'i', but fourth declension singular nominative nouns also end in 'us', but their plural form still end in 'us'. Also, like in every language, there are funky exceptions to these rules, like second declension singular nominative nouns which are neuter rather than masculine, but still end in 'us' rather than the normal 'um'. Moral of the story, don't assume that the plural of word ending in 'us' is 'i'.

It's also been about 8 years since I've taken Latin, so take that into consideration before someone goes all Life of Brian on me.