Latin neuters end in "-a" in the plural for the nominative and accusative cases.The plural of virus is viruses in English -- at least at the moment. 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.
Datum > Data
Singular > Plural
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.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.
Alumnus > Alumni
Singular > Plural
Although viruses is probably the best plural for virus, while viri is 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.
A Related Note: OctopusOctopus 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.
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