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However wrong it may be, lots of people have pluralized virus as virii. I'd understand viri, but what misconception could lead one to write virii?

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My guess is that radius, radii was generalized to virus, virii. People seem to have not noticed that the word is virus and not virius. – Peter Shor Apr 23 '11 at 19:51
@Peter: Thanks, that sounds reasonable. – user4727 Apr 23 '11 at 20:03
@Peter Shor: You should probably write that as an answer. – Kosmonaut Apr 23 '11 at 20:34
Anyone who has ever said “virii” is so dumb. Really, really dumb. The plural is “virus”, since we don't even know what declension it was originally from. Some people think that it was 4th declension, which would make the plural also “virus”, but we really don't know. Just say “viruses”. Oh my goodness. – user3217 Apr 25 '11 at 0:38
Please read this, so I don’t have to copy it all into an answer. Again. – tchrist Aug 19 '12 at 3:42
up vote 4 down vote accepted

I suspect the confusion arises out of the fact that very few people now have any kind of understanding of Latin. Many words get turned into faux-Latin plurals, even when the word itself is not Latin in origin, or when the plural form is completely incorrect (octopi, platypi, penii). It seems to come up most often when the plural form of the word has an -ses construction. It feels uncomfortable to say, so people (often jokingly, in my experience, but occasionally earnestly) try to give it a faux-Latin plural.

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On the Italian Wikipedia page for Virus, it seems that there are different plurals used in different contexts.

I can't quote it because it's italian and almost no-one will understand so I'll try to translate the important parts:

It should be singularia tantum (=it only has the singular form), like "rice" or "air".

Lwoff, Horne and Tournier, within their classification in 1962, proposed and used the form "vira". In the anglophone area of interest, virii (from vīriī) is rather used referring to Computer viruses, while within biological area the one used is "viruses".

Vīriī is wrong because it would come from a word *vīrius (such as radius, radiī) which does not exist.
Another plural could be vīrī which instead would require the singular form "vir" that means man and whose plural form is "vĭrī".

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+1 Note that the plural would be viri in Latin if it existed (quod apparently non), since the genitive viri, which exists, shows that it is of the second declension. – Cerberus Apr 23 '11 at 20:00
A direct translation can be found on English Wikipedia's Plural forms of words ending in -us. I was hoping to see a theory on why virii became so wide-spread. As @Cer said, such a Latin plural would be viri. – user4727 Apr 23 '11 at 20:02
Yes, I actually found the declension of the word Virus, in Latin, and it had the plural form written, so I guess it just got out of use? – Alenanno Apr 23 '11 at 20:09
Having worked with and around computers and programmers most of my life, I can safely call BS on the notion that "virii" is used with respect to computer viruses. – The Raven Apr 23 '11 at 20:49
@Alenanno: It means the solid excrements of a male cow. – Cerberus Apr 23 '11 at 22:10

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