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.iri 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. Octopus comes from the Greek, so the-us ending does not mark a Latin masculine noun of the second declension.