I felt really silly after dutifully consulting both a pocket paper dictionary and an on-line one and blithely accepting that "n.". After a while I realised I had no idea at all whether it stood for noun or neuter. In my paper dictionary the adjectives are marked a. but some nouns are marked f. -- not n.f. just f. So I think it's saying virus is neuter, not just a noun.
or the assumption was made that it was second dec and fitted the "normal" formation for plurals
But this is the very question at issue. What is the "normal" plural for a subclass where no normal examples are known? There are no normal second declension neuters with plurals! Very careless of them, I must say. It's not as if they're semantically ruled out, is it? If Greek words like whale and sea can fit in, and they have plurals, why shouldn't Latin have had them? Or, on the other hand, what on earth is a 2nd.decl. neuter doing with the ending -us?
But my claim is that he "normal" plural would have to be -a, if it's neuter.
