I agree 100% it should be viruses, nothing else.

It is an interesting question what it would have been in Latin. Did they ever write aquae, lactes? From the look of it, if it was neuter it would have to have taken -a, and I can't see anything in the stem except vir- (there's an adjective vir-osus meaning slimy), so it would have to be vira if they'd chosen to make one. It's a long i, so it's different from vir = man.