Fail!
Mass noun in Latin
Virus comes to English from Latin. The Latin word vīrus (the ī indicates a long i) means "poison; venom", denoting the venom of a snake. This Latin word is probably related to the Greek ἰός (ios) meaning "venom" or "rust" and the Sanskrit word visham meaning "toxic, poison".[2]
Since vīrus in antiquity denoted something uncountable, it was a mass noun. Mass nouns — such as air, rice, and helpfulness in English — pluralize only under special circumstances, hence the non-existence of plural forms in the texts.[3]
It is unclear how a plural might have been formed under Latin grammar if the word had acquired a meaning requiring a plural form. In Latin vīrus is generally regarded as a neuter of the second declension, but neuter second declension nouns ending in -us (rather than -um) are so rare that there are no recorded plurals. Neuter nouns of other declensions always end in -a (in the nominative, accusative and vocative), but even if we were to apply this rule to vīrus, it would be conjecture to guess whether this should give us vīra, vīrua, or something else. There simply is no known plural for this word in Classical Latin.
In Neo-Latin, a plural form is necessary, in order to express the modern concept of ‘viruses’. Dictionaries such as Whitaker's Words therefore treat it as a second-declension noun with the following fairly ordinary forms:
singular plural
nominative vīrus vīra
vocative vīrus/vīre vīra
accusative vīrus vīra
genitive vīrī vīrōrum
dative vīrō vīrīs
ablative vīrō vīrīs