The correct English plural of virus is viruses.
The following comes from usenet and verified by one professor, but not without some comment [see below].
For the purists, in Latin, there is a rarely-used plural form:
virus, viri (neuter)
(Forms: almost always restricted to nominative and accusative singular; generally singular in Lucretius, ablative singular in Lucretius)
The point of this is that even in Latin the form “viri” is rarely used. The singular form is used in most every instance. (This is from the Oxford Latin Dictionary.)
Comment received on the above: This is incorrect. There is NO example in classical texts of the use of the incorrect *viri. Also, if it were used as in Lucretius, the proper plural would STILL not be *viri, but rather *vira or possibly virora. If, as some believe, it were 4th declension, rather than 2nd, the proper plural wold simply be “virus,” with the short-u changed to a long-u in pronunciation. Either way, however, there is NO example of the use of the word “virus” in a plural context prior to the modern era. [From: MC]
So, when considering the Latin: “virii” is incorrect and “viri” was almost never (or never if the comment is correct) used.
Despite the fact there was little use for the plural form, there is another reason why “viri” was rarely used. The most common Latin word for “man” is “vir” with “viri” being its plural in the form used as the subject of a sentence. Thus, since “men” as the subject of a sentence would be used far more often than “venoms” (virus means venom) the “viri” word was most commonly seen as the plural of “man.”
Bottom line: Don’t try to make up words using a false Latin plural form. Since the word virus in its English form is now used then the English plural, viruses, should be used.
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Comments from original:
William
Said this on 2009-06-17 At 11:28 am
The comment above is correct. Although I haven’t read ALL classic literature and can’t validate that part.
Viri equals men, or the plural form of vir ( man ).
Viruses is the proper plural form for virus.
#2
Knut Holt
Said this on 2010-02-23 At 05:36 am
The regular plural of virus would be virora. Viri is incorrect. Viruses is probably correct in English usage.
It seems however that the form vira is used pretty much, and it sounds better than viruses. It has a correct ending, but the stem form is not quite correct.
So viruses and vira seems both to be acceptable.
[No English dictionary I’ve seen confirms vira as a proper plural form in English no matter how it sounds. –DaBoss]