Actually, the plural of "vir" is "viri"; Latin "virus" doesn't really have a plural but if it did it might be "via"; "virii" isn't the plural of anything; if it were, it would be of "virius" which I'm pretty sure isn't a Latin word at all. (If there were an adjective "virus" its comparative would have "virius" as one of its forms, but it wouldn't become "virii" in the plural, and in any case I don't think there is any such adjective.)
Wiktionary shows it has "virī", with a long i. I guess the double i is just a way to write the long i when you don't have macrons at hand, but I have never studied Latin, so I don't really know.
