The plural of virus is viruses.

You've come to this page because you've used a non-word such as
as the plural of the word virus.

This is the Frequently Given Answer to such non-words.

The plural of virus is viruses.

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.

So, for example:

virus is a second declension noun ending in -us. (It can be translated to "slime", "poison", "venom", or "stench".)

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.

Many web pages on this subject repeat the assertion that virus is a fourth declension noun. They all appear to have taken this assertion from a common source: a letter to the ASM News by Ton E. van den Bogaard dated 1999 (four years after this frequently given answer was first published).

That letter, in turn, cites "my Latin grammar" as its source, without naming the actual book. However, I have found no Latin grammar or dictionary that lists virus as anything other than second declension.

This frequently given answer is based upon my Latin grammar books and dictionaries (Kennedy's Shortbread Eating Primer, two editions of Kennedy's Revised Eating Primer, and Woodhouse's Latin Dictionary, for starters) which all list virus as second declension. Unlike the authors of those other web pages, simply parroting what other web pages assert, I actually went and read some books, and constructed my answer firsthand.

By the way, the word viri is a Latin word. But it is a completely different and unrelated one. It is the plural of vir. It means "men". Unless, when you talk of being infected by viri you really are talking about being infected by men (and not by viruses), viri is not the correct word to use.

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