Dictionary.com Word FAQs
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.