1) The Latin plural of word ending in -us is not -ii. -i at best.
2) 'Virus' doesn't have a Latin plural, because its meaning is like (in the sense of not having a plural) 'sand': it already denotes a multitude.
"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".
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
A bit hard to communicate, but that's the keyboard cat playing all of you off.
As an amateur Latin geek myself, I agree that "viruses" is proper from a grammar standpoint, but I sided with Wikipedia because I was using computer terminology.
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.
Put it like this: Grammatically speaking, the plural of virus is viri. Putting it into plural might or might not makes sense. Personally, I don't think that using plural for collections in Latin is a very big sin given that this is very common in classical Latin texts.
One example of this can be found in the famous introduction of Aeneid (I.1 "Arma virumque cano...") lines 31-32, where Virgil is using the plural form of the word "sea" (mare, plural: maria)
"multosque per annos / errabant acti fatis __maria__ omnia circum" - "for a number of years, driven by fate, wandering around on the seas"