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?

Neuter and Masculine Nouns

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

"Virus," by the way, can refer either an "infectious agent" or a computer program that "is usually disguised as an innocuous program or file," which inserts a copy of itself into another program "that when run usually performs a malicious action," Merriam-Webster notes.

The Plural of Octopus

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."

Sources

• “Virus.” Merriam-Webster.com, Merriam-Webster..