And we certainly don't grab for genitive singulars for the plurals when we've started out with a nominative. Such hanky panky would certainly get you talked about, and probably your hand slapped as well. This apparently invariant use of virus as a genitive singular may also imply that it's 4 th declension, as some scholars believe. The crucial problem here is that, classically speaking, there appears to be no recorded use of virus in the plural. It was a 2 nd declension noun ending in -us , which is rather common, but it was also a neuter, which is rather rare.
None appear to admit plurals.
Perhaps this is because they are mass nouns, not count nouns. Most other citations think that these plurals just never happened at all, or that if they did, they didn't jump declensions. Perhaps they were invariant as they oddly are for the vocative and accusative cases. And it really seems to do neither. So what we have here is something of a mixed or invariant declension.