"Another theory holds that virus, being a 2nd declension neuter--which we are 100% certain of because its nominative singular is -us and its genitive singular is -i--must go to *vira in the plural as do its -um neuter brethren in the 2nd declension. However, that assumes that it works like a -um form, not as a -us form does. And it really seems to do
neither. If it were a -us form (again, as a 2nd declension nominative), then its vocative would have to be *vire; but it's really only virus. You also expect an accusative form *viros, but that too is missing; it's still just virus in the accusative. And if it were a -um form, then its vocative would have to be *virum. But it's not--here again, it's only virus. (Vocative examples of virus are not particularly common. Apparently the Romans seldom addressed their slime in a personal fashion. :-)" It all starts to sound like
new math to me.