Is the plural of virus "viruses" or "virii"?

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    Definitely not "virii" — the only plurals that end in '-ii' are those of words ending in '-ius'. "Virii" could only be the plural of 'virius', and only under the right circumstances. Oct 8, 2010 at 19:39

2 Answers 2


Wikipedia has a rather large section dedicated to this question. The verdict is: “The English plural of virus is viruses, not viri.” Merriam-Webster agrees, as does TheFreeDictionary. Wiktionary offers the following usage notes:

The plural is often believed to be viri or even virii, but neither is correct Latin and both are neologistic folk etymology. The word has no plural in Latin as it is a mass noun, like oxygen or sunlight.

  • 3
    +1. Plus we are free, in English, to form plurals in the English way if we want :) Oct 8, 2010 at 19:44
  • @Mr. Shiny and New: I wholeheartedly agree, as does that Wikipedia article: "Even if the Latin plural were known, English speakers would not be obliged to use it". (It even proceeds to expressly mention campus and bonus.)
    – RegDwigнt
    Oct 8, 2010 at 19:52
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    I guess I don't understand the distinction between a "known" plural and a "used" plural. English speakers -- who are not Latin speakers -- don't "know" declensions of Latin nouns. What they know is productive morphology in contemporary English for forming plurals. What are the "known" plurals of these words? Kindergarten-opera-blitzkrieg-tsunami-tycoon-phobia. All of these are foreign borrowings, which have "known" plurals in their respective languages of origins. But we don't pretend in English to be importing both the word and its plural forms. So why do we with Latin words?
    – Mike Pope
    Oct 8, 2010 at 20:48
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    @Mike Pope: smbc-comics.com/index.php?db=comics&id=1981
    – Claudiu
    Oct 8, 2010 at 22:14
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    @Mike Pope: English has certain ways to form plurals. But sometimes the word AND its plural WERE imported by the early users of the term. Consider a medical term imported or coined from Latin. If the medical community imports (or coins) the word AND its plural together, and use the Latin forms in English contexts, then we have two new English words. When those words escape medical-jargon into the language as a whole, then the plural will be a Latin plural. Oct 12, 2010 at 14:53

Speaking as a descriptivist, I would have to say that "virii" does indeed exist, in English rather than bad Latin, but only as a whimsical or jocular form (and particularly in an IT context). Strictly "viruses" for an unmarked usage.

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    Some of those uses are whimsical or jocular and some are hypercorrection. I know I used to say "viri" (and write "virii") before I learned that I didn't need to do that to be "correct". Oct 12, 2010 at 14:55
  • Fair comment. I think I first encountered it in a job lot with e.g. boxen and Elvii (cf the jargon file/hacker's dictionary), so the wilfully whimsical style of use was fairly clear from that - I shouldn't universalise my own experience. Oct 13, 2010 at 20:54

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