"Virus" is also a word that frequently gets pluralized incorrectly, if you're thinking of it as a Latin word used in an English context. The only plural form that makes sense in English is "viruses," mostly because the Latin form actually
has no plural. [0]
That doesn't stop people from writing "virii," however. Both "octopi [1]" and "virii" are examples of linguistic hypercorrection [2], in which a speaker misapplies a known linguistic rule that seems to fit, but does not. Neither of these examples are attested in the source language the word derives from; people literally use them just to sound smart, most of the time.
Other examples are using "whom" in places where the correct word would be "who," not using a preposition to end a sentence or clause with [3], and to not split infinitives [4].
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[0]: https://en.wiktionary.org/wiki/virus#Etymology
[1]: https://en.wikipedia.org/wiki/Hypercorrection
[2]: I kinda like "octopodes" myself, but that may just be me.
[3]: Winston Churchill is frequently quoted saying something like "this is the kind of nonsense up with which I will not put." This is most likely apocryphal. See https://quoteinvestigator.com/2012/07/04/churchill-prepositi...
[4]: My favorite origin theory for this one is basically "you couldn't do it in Latin, so don't do it in English." Early grammarians frequently took inspiration from Latin, because its grammar was already well-codified, mostly due to it being a dead-ish language. See https://en.wikipedia.org/wiki/Split_infinitive#The_argument_...