Typo

Your Wrong Velthur

The plural of virus is neither viri nor virii, nor even vira nor virora. It is quite simply viruses, irrespective of context. Here's why.

English Inflections
First off, the OED gives nothing but viruses for the plural. Here's its abbreviated entry:

Etymology: a. L. virus slimy liquid, poison, offensive odour or taste. Hence also Fr., Sp., Pg. virus.

1 Venom, such as is emitted by a poisonous animal. Also fig.

2 Path. a A morbid principle or poisonous substance produced in the body as the result of some disease, esp. one capable of being introduced into other persons or animals by inoculations or otherwise and of developing the same disease in them. Now superseded by the next sense.

b Pl. viruses. An infectious organism that is usu. submicroscopic, can multiply only inside certain living host cells (in many cases causing disease) and is now understood to be a non-cellular structure lacking any intrinsic metabolism and usually comprising a DNA or RNA core inside a protein coat (see also quot. 1977). [ Formerly referred to as filterable viruses, their first distinguishing characteristic being the ability to pass through filters that retained bacteria. ]

Other sources that support viruses include Birchfield (né Fowler :-) in Modern English Usage (3rd Edition), and also the Cambridge Encyclopedia of the English Language.
Classical Inflections
While one would hope that the authoritative sources cited above would suffice, some 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. And although there actually is a viri form for virus, it's the genitive singular[1], 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[2], 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. [3]

One citation below wonders whether these -us 2nd declension neuters might have inflected -us => -ora, the way the 3rd declension's neuter plurals for tempus and corpus do. There's really not any support for that notion--that I could find at least. If so, that would end up producing *virora. Most other citations think that these plurals just never happened at all, or that if they did, they didn't jump declensions. Perhaps they were invariant as they oddly are for the vocative and accusative cases. In any event, *virora does not fit comfortably in the mouth of an English speaker, which is a good reason to avoid it.[4]

Another theory holds that virus, if it was a 2nd declension neuter, must go to *vira in the plural as do its -um neuter brethren in the 2nd declension. However, that assumes that it works like a -um form, not as a -us form does. And it really seems to do neither. If it were a -us form (again, as a 2nd declension nominative), then its vocative would have to be *vire; but it's really only virus. You also expect an accusative form *viros, but that too is missing; it's still just virus in the accusative. And if it were a -um form, then its vocative would have to be *virum. But it's not--here again, it's only virus. (Vocative examples of virus are not particularly common. Apparently the Romans seldom addressed their slime in a personal fashion. :-)

So what we have here is something of a mixed or invariant declension. Trying to find a plural for something that didn't take a plural (possibly because it was not a count but a mass noun), or at least, one for which no plural is classically attested, is a fruitless endeavour. Best to stick with English and use viruses.

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