The word virus has no classically attested plural form in Latin. In antiquity the word had not yet acquired its current meaning. It denoted something like toxicity; venom; a poisonous, deleterious, or unpleasant agent or principle; or poison in the abstract or general sense[2]. Nouns denoting countable entities (such as book) pluralize; noncountable entities (such as air, mood, valor) pluralize only under special circumstances. The term virus in antiquity appears to have belonged to the latter category, hence the nonexistence of plural forms. [June 1999 issue of ASM News by the American Society for Microbiology]
It is unclear how a plural might have been formed had the word acquired a meaning requiring a plural form. Possibilities include vira, following the pattern for neuter nouns in -um or virus with a long , following the example of status. However, none of these are attested. The virii form would not have been a correct plural, since the -ii ending only occurs in the plural of 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 viri form is also incorrect in Latin. The ending -i is used only for masculine 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".