The form virii would not have been a correct plural, since the ending -ii only occurs in the plural of masculine and feminine words ending in -ius. For instance, take radius, plural radii: the root is radi-, with the singular ending -us and the plural -i. Thus the plural virii is that of the nonexistent word virius . The form viri would also be incorrect in Latin. The ending -i is normally used for masculine or feminine nouns, not neuter ones such as virus; moreover, viri (albeit with a short i in the first syllable) is the plural of vir, and means "men."