The Plural of Virus

It is common knowledge that the plurals of many words from Latin end in "-a" or "-i." Data, for instance, is the plural of datum and alumni is the plural of an alumnus. Is the plural of virus viri and if not, why?

​Latin neuters end in "-a" in the plural for the nominative and accusative cases:

• Datum > Data
• Singular > Plural

The plural of virus is viruses in English—at least at the moment. The virus is a neuter noun in Latin. That means its plural, if there were an attested ancient usage of virus in the plural, would have ended in an "-a," because of neuter nouns in (ancient Greek and) Latin end in an "-a," in the plural nominative and accusative cases. The example of the plural of datum is a case in point. Since datum is a neuter singular, its plural is data.

Since the virus is neuter, vira is a possibility for the nominative/accusative plural. It could not be viri.

Second declension masculine nouns end in "-i" in the nominative plural

• Alumnus > Alumni
• Singular > Plural

Viri is the plural of the masculine 2nd declension noun vir, which means 'man'. Vir 'man' is a masculine noun and the "-i" ending is appropriate for the plural nominative of masculine second declension nouns.

Although viruses are probably the best plural for the virus, while viri are currently wrong, languages change. Because of the frequency with which viruses threaten the Internet, and the resultant frequency of articles using various endings on the word virus, we may have a new standard plural for virus before we know it.

Octopus comes from the Greek, so the -us ending does not mark a Latin masculine noun of the second declension. The Greek-based plural is octopodes, but like other words taken into English, an -es ending on the singular (octopus > octopuses) is acceptable. Octopi is wrong for the plural of octopus, like viri for the plural of virus.