[adrian_b]

All the attested uses of the word virus in Latin are only in the singular number.

Nevertheless, the correct plural would have been "virora", like tempus => tempora (time => times) or corpus => corpora (body => bodies), or perhaps "virera", like pondus => pondera (weight => weights) or genus => genera (kind => kinds), depending on the original quality of the final vowel in the stem.

(Originally it would have been visos => visosa, but the final vowel in virus has become closed, while the intervocalic s has become r due to rhotacism.)

You have been thinking at the masculine words whose stem ends in -u, like fructu (fruit), where the singular is fructus and the plural is fructuus. There -s is not part of the stem but it is a marker of the singular masculine nominative case.

In virus and the other neuter nouns that end in -s, the -s is a part of the stem of the word, not a case marker. There are also masculine word where the stem ends in -s, like muus (mouse), in which the -s must also not be confused with the marker of the nominative case that is applied to some of the words with other kinds of stems.

[DFHippie]

> the correct plural would have been "virora", like tempus => tempora

This would be true if virus were a third declension noun. It it is second declension noun. Its genitive is viri, not viroris.

[adrian_b]

It is true that there are a few cases of a word "viri", which might have been the genitive of "virus".

Nevertheless, the meaning of the word is not certain, at least in the examples that I have seen.

Even if the word "viri" was really intended as the genitive of "virus", that is just another example of many cases when even the native speakers of Latin were not certain about the gender and declination class of certain seldom used words.

Whoever has used "viri" as the genitive of "virus" was believing that it is a masculine word of the 2nd declension. Most attested uses of virus are consistent with it being a neuter of the 3rd declension (i.e. "virus" was used for the accusative case). The word virus cannot be a neuter of the 2nd declension (in that case it would have been "virum").

Actually it is possible that in very old Latin the word virus was indeed a masculine of the 2nd declension, like its cognate word in Greek, but due to its meaning as a name of a substance it was transferred to the neuter gender in the 3rd declension.

Such interconversions of the words ending in -us between 2nd declension masculine, 3rd declension neuter and 4th declension masculine have happened for many words during the history of the Latin language, because even some native speakers guessed wrong the word class after hearing a rare word just a few times, and then others imitated them.

[DFHippie]

That all makes sense. What I know of virus is all from books like Allen and Greenough's New Latin Grammar, which is basically a 19th century understanding: languages have particular rules one can enumerate and deviations from these rules are errors.

[thaumasiotes]

That understanding is basically correct. But the history of Latin is quite long, and the rules change over time.