Over the weekend, I temporarily lost access to both of my computers, apparently due to computer ... viruses. Or is it "vira"? Tom Christiansen gives the matter its due attention here:
[S]ome writers prefer to maintain the classical inflections on some English words, particularly in technical writing. For example, conflicting indexes/indices and minimums/minima are both easily found, depending on the intended audience and use. In that case, what's the classical plural of virus?
The simple answer is that there wasn't one. The longer answer follows.
Writers who, searching for a fancy plural to virus, incorrectly write *viri are doubtless blindly applying an overreaching -us => -i rule. This mis-inflects many words. For example, status and hiatus only change the length of the final vowel; genus goes to genera; corpus goes to corpora. Others are even worse if this rule is mis-applied, like syllabus, caucus, octopus, mandamus, and rebus.
Anyway, Latin already had a word viri, but it was the nominative plural not of virus (slime, poison, or venom), but of vir (man), which as it turns out is also a 2nd declension noun. I do not believe that writers of English who write viri are intentionally speaking of men [though that may be debatable - AGG]. And although there actually is a viri form for virus, it's the genitive singular, not the nominative plural. And we certainly don't grab for genitive singulars for the plurals when we've started out with a nominative. Such hanky panky would certainly get you talked about, and probably your hand slapped as well.
This apparently invariant use of virus as a genitive singular may also imply that it's 4th declension, as some scholars believe.
Those confused souls who write *virii are tacitly positing the existence of the non-word *virius, and declining it as though it were like filius. It's true that l/r are both linguals that sometimes get interchanged, and that f/v are just a change in voicing, but that's just reaching. *Virii is still completely silly, so don't do that; otherwise, everyone will know you're just a blathering script kiddie.
The crucial problem here is that, classically speaking, there appears to be no recorded use of virus in the plural. It was a 2nd declension noun ending in -us, which is rather common, but it was also a neuter, which is rather rare. I could only come up with three such 2nd declension neuters: virus (some poison), pelagus (the sea, usually poetically), and vulgus (the crowd). None appear to admit plurals. Perhaps this is because they are mass nouns, not count nouns.
Ton E. van den Bogaard of the University Maastricht in the Netherlands follows up with a letter that firmly places him in the camp of the fourth declensionites:
With interest I read the contribution `On the Absence of a Plural of the Latin Noun ``Virus' in the June 1999 ASM News, p. 388, by Robert J. Smutny. However, according to my Latin grammar, one of the very few books of my gymnasium (high school) days that is still up to date, the plural of the noun virus in Latin is, like the plural nowadays used for virus in Romance languages (e.g., Italian and French), also virus. The Latin noun virus does not belong to the second declension group but, like the noun fructus, meaning fruit or piece of fruit, belongs to a group of Latin words that is declined according to the fourth declension. Hence, two pieces of fruit is in Latin duo fructus and two viruses would be duo virus. According to the fourth declension the plural genitive of virus in Latin is viruum and therefore an Index of Viruses is in Latin an Index Viruum. Virorum is the plural genitive of the Latin noun vir (second declension) meaning man or husband. Consequently an Index Virorum would indicate a list of husbands or men.
Moreover, because the noun virus belongs to the fourth declension group the study of viruses should have been called virulogy and people practicing that science virulogists. My former professor in virology at veterinary school consequently called himself a virulogist and he lectured virulogy. I am afraid that these words have become extinct since he died.
It is important to realize that Latin and Greek derived expressions in biomedical English have been coined by scientists for convenience and not by scholars based on classical grammar. The old Romans might have said to these scientists modulating their language: ``Ut desint vires, tamen est laudanda voluntas,' which means freely translated: ``Despite your lack of knowledge, still appreciated.'
Yes, but how would the old Romans have translated "blathering script kiddie"?