>virus is by Latin definition a masculine noun<

Um. I learned Latin at school for (I think!) six years. LIke wsieber, I think you need to be careful to distinguish between the gender of a noun and its declension.

I also think you need to be aware that there are five different regular declensions of nouns in Latin. (I refuse to explore the question of exceptions to formal rules, since I don't remember enough!) Important in this instance is that two of these declensions have nominative singular ending in -us. So it is impossible to determine the plural from this nominative singular ending alone.

Like wsieber, I searched the web. (Those six years were a long time ago...) I typed 'latin noun' into yahoo.com, and the first match that came up was titled 'What's the plural of virus?' Synchronicity? Or just affected by everyone else having searched for it recently? Anyway, the link:

http://www.perl.com/pub/language/misc/virus.html

You can also follow the links on the first page of search results to get full details of the five declensions in all cases.