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. 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 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 the native speakers guessed wrong the word class after hearing a rare word just a few times.

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.

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