The Plural of Virus

The word 'VIRUS' spelled with blocks on a wooden table
Daniel Sambraus / Getty Images

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."


  • Virus.”, Merriam-Webster..
mla apa chicago
Your Citation
Gill, N.S. "The Plural of Virus." ThoughtCo, Jul. 26, 2021, Gill, N.S. (2021, July 26). The Plural of Virus. Retrieved from Gill, N.S. "The Plural of Virus." ThoughtCo. (accessed March 22, 2023).