From Wikipedia:
Virus
The English plural of
virus is
viruses.
[1] In most speaking communities this is non-controversial and speakers would not attempt to use the non-standard plural in -i. However, in computer enthusiast circles in the late 20th century and early 21st, the non-standard
viri form (sometimes even
virii) was well-attested, generally in the context of computer viruses.
[2]
While the number of users employing these non-standard plural forms of
virus was always a proportionally small percentage of the English-speaking population, the variation was notable because it coincided with the growth of the
Internet, a medium on which users of
viri were over-represented. As the distribution of Internet users shifted to be more representative of the population as a whole during the 2000s, the non-standard forms saw decline in usage. A tendency towards prescriptivism in the computer enthusiast community, combined with the growing awareness that
viri and
virii are not etymologically supported plural forms, also played a part.[
citation needed]
Nonetheless, the question of what the Latin plural of
virus would have been turns out not to be straightforward, as no plural form is attested in extant Latin literature. Furthermore, its unusual status as a neuter noun ending in -us apparently not of Greek origin obscures its morphology, making guesses about how it should have been declined difficult.[
citation needed]