Never understood using "virii" as a plural of "virus. Figured "viri" made some sense, but guess that would actually mean "men". Only just now got around to googling it. Not what I expected to find. Don't recall such a thing as a "neuter noun" - at least not one that ended in "us".
From
What Is 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.