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