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Etymology: a. L. virus slimy liquid, poison, offensive odour or taste. Hence also Fr., Sp., Pg. virus.Other sources that support viruses include Birchfield (né Fowler :-) in Modern English Usage (3rd Edition), and also the Cambridge Encyclopedia of the English Language.
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 Now superseded by the next sense.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. ]
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, 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. 
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.
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.
qui ut coluber copia virus exuberans natorumThat seems to be using virus as a genitive, which contradicts the assertion that it's 2nd declension, which would have lead to viri, and supports the 4th declension position. This was brought to my attention by Andreas Waschbuesch, who went on to write:
Just another note: You must not forget that Ammian's native tongue was Greek, not Latin - so it's (very hypothetical!) possible he understood virus as a so called accusativus respectus and copia as adverbial expression. (A more common phenomenon in Greek.) exuberare was combined that way with lucrum and there was a tendency to use non-transitive verbs in a (active) transitive way - like anhelare or spumare in late antiquity's Latin as well. (The pseudo-Ciceronian Rhetorica ad Herennium's fourth book is an outstanding exception with its usage of anhelans et spumans in the passage about the denarratio and the following example IF one dates it to 80 a.Chr.n. ...) But - to make a conclusion - it's not classical at all to use the form viri(i), because there isn't any genitive-singular- or nominative-plural-form (*) viri found in the whole Latin literature up to the first century p.Chr.n. as far as PHI-CD-Rom can tell :-)This recent letter also supports the fourth declension point of view. Of course, even if virus really turns out to have been in the fourth declension, we'll still have vulgus, pelagus, and cetus as irregular -us neuters in the second declension. Let's blame it all on the Greeks.
Here's what other sources have to say about this matter:
Latin had several declensions containing neuter, feminine, and masculine words ending in -us; the plurals are different in each one. Incidentally, the singular of mores (pronounced `moh-rehs') is mos, with the same change of `s' to `r' between vowels heard in corpus : corpora and in genus : genera.
Many Greek nouns retain their original gender: as, arctus (F.), the Polar Bear; methodus (F.), method.Whether this leading would lead to ?vire, however, is unclear, since virus does not appear to be of Greek extraction.
a. The following in -us are Neuter; their accusative (as with all neuters) is the same as the nominative: pelagus, sea; virus, poison; vulgus (rarely M.), the crowd. They are not found in the plural, except pelagus, which has a rare nominative and accusative plural pelage.
NOTE.--The nominative plural neuter cete, sea monsters, occurs; the nominative singular cetus occurs in Vitruvius.
/* Begin Excerpt */
Numerous Latin words have been taken over into the modern scientific vocabulary, most without difficulty. The Latin word virus, however, presents a minor but interesting problem, if one wishes to express a phrase such as Index of Viruses in its Latin form. By analogy with other nouns, one would expect the normal Latin equivalent to be Index Virorum. The difficulty stems from the fact that the Latin noun virus is defective, i.e. does not have a full set of case--forms, singular and plural. The Roman grammarian Priscian (fl. 500 A.D.) states that some claim the word is indeclinable (i.e., has only one form for all the cases in the singular); others, apparently more accurately, that it is declined in the singular according to the second declension neuter and cite two passages from the poet Lucretius in substantiation. All of the ancient grammarians are in agreement, however, that the word is used in the singular only, which indeed appears to be true, for no plural forms are attested in extant Latin works./* End Excerpt */
In antiquity the word virus had not yet acquired, of course, its current scientific meaning; rather it denoted something like toxicity, venom, a poisonous, deleterious, or unpleasant agent or principle, or poison in the abstract or general sense. (The first meaning given for this word, a slimy liquid, slime, in the most widely used Latin-English dictionaries is inaccurate; the error has been corrected in the more recent Oxford Latin Dictionary.) Nouns denoting entities that are countable pluralize (book, books); nouns denoting noncountable entities do not (except under special circumstances) pluralize (air, mood, valor). The term virus in antiquity appears to have belonged to the latter category, hence the nonexistence of plural forms.
When the word was taken over into modern languages and acquired its current scientific meaning, it changed categories and denoted a countable entity. The modern languages which have adopted the word each pluralize it in their own fashion (e.g., Eng. viruses, Germ. Viren; French and Italian do not distinguish in form between singular and plural, virus). But what to do in neo-Latin, which normally is subject to the rules and constraints of classical Latin?
W. T. Steam in his manual on botanical Latin (Botanical Latin, Newton Abbey, 2nd ed., 1973) gives what would be the normal plural forms of such a second declension neuter noun: nominative vira, genitive virorum, without, however, indicating his authority for those forms. It may be observed that in Latin as in other languages when the plural of noncountable nouns does occur, it generally denotes various kinds of the entity (e.g., wine, honey, oil). Steam may have applied this principle to virus in order to meet the requirements of modern scientific terminology. If Latin had continued to be the common international language of scholars and scientists at the time that viruses were first identified, it appears likely that it would have generated the forms adduced by Steam.
Robert J. Smutny
On the Presence of a Plural of the Latin Noun "Virus"
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.''
Ton E. van den Bogaard
University Maastricht, the Netherlands
Here are some Web resources:
% cd /tmp % wget -r -l2 http://humanum.arts.cuhk.edu.hk/Lexis/Wheelock-Latin/ % netscape /tmp/humanum.arts.cuhk.edu.hk/Lexis/Wheelock-Latin/index.html
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O tempora, o mores! Senatus haec intellegit. consul videt; hic tamen vivit. Vivit? immo vero etiam in senatum venit, fit publici consilii particeps, notat et designat oculis ad caedem unum quemque nostrum.
Cicero, Oratio in Catilinam Prima, 2
Last update: Wed Nov 17 09:20:10 MST 1999