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What is the plural of virus?

Viruses. It is not viri, or (worse) virii. True, the word comes directly from Latin, but not all Latin words ending in -us have -i as their plural. Besides, viri is the Latin word for 'men' (plural of vir, 'man', the root of English virile). There is in fact no written attestation of a Latin plural of virus. If you would like to pursue the subject further, see the excellent article "What's the Plural of Virus?" at http://www.linuxmafia.com/~rick/faq/plural-of-virus.html. The convention for forming the plural of Latin words in English is to use the Latin plural form, or, if Latin does not actually have a plural form for the word, to form the plural in the normal manner used for other English words. Virus is a second declension noun ending in -us. However, it is one of the few such nouns that has no plural in Latin. It occurs only in the singular. So, just as with ignoramus, one forms its English plural by appending -es.

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