Oh, as I said in my first response in this thread, in English it should undoubtedly be viruses. Can you imagine a doctor saying "Well, your offspring has come down with one of two or three candidate viri?" I think not ...
But it's an interesting discussion nonetheless.
Bingley, the Notre Dame entry specified virus -i, which means it's (a) second dec and (b) either masculine or feminine. The "n" in their dictionary is the part of speech - noun.
I would imagine that it pretty much fitted into the class of uncountable nouns, although there must have been occasions when a plural was necessary. In that case I would imagine that either the singular form was used in a plural context in the same way we use "sheep" as both singular and plural, or the assumption was made that it was second dec and fitted the "normal" formation for plurals. Or some completely different word was substituted, I suppose.
