This is actually discussed in an old Straight Dope column, if you don’t want to believe my high school Latin memory.
The plural of “virus” is not “viri,” nor is it “virii.” “Viri” is a Latin word, but it means “men.” It’s the plural of the word “vir” (the root of “virile,” if anyone cares). “Virii” does not exist. If it did, it would be the plural of “virius,” but virius is not a word.
“Virus,” in classical Latin, meant “slime.” In particular, it meant “poisonous slime,” or in general “stuff you DO NOT want to touch.” It also meant “animal venom.” As such, it was a mass noun, like butter, not a counting noun, like brick, and did not take a plural. A word like “butters,” to mean “varieties of butter” is a recent linguistic invention in English, and would not make sense to a Roman.
“Virus,” to mean “infectious RNA particles that cause illnesses” is an English word. It is a loan word from Latin, but the meaning in English is different, and it takes an English plural, so “viruses,” means “different kinds of such.” To mean individual virus particles, I would say “particles of virus,” but I am not a virologist. There may be a term, or they may say “the viruses,” or something. I’d have to ask my cousin.
My degree is in English, though, and I am sure of the fact that words as divorced from their origin as “virus” are pluralized as English words. Compare, for example, the English plural “operas,” to mean “several musical compositions for staging with singers in costume who act out parts.” This is what an “opera” is in English, and it’s a singular word. In Latin, it’s a plural. Of opus. The plural of opus is not “opi.” Now, in English, the plural of opus, is, of course, opuses.
You will not hear from me if you pluralize ANY loan word as an English word. You can say cherubs, and not cherubim; you can say dogmas, and not dogmata; you can (and really SHOULD) say bagels, even though in Yiddish, bagel is like deer and fish-- the singular and plural are the same. Also, please say babushkas, and not babushky, as they are in Russian-- this is another word that means something entirely different in English as it does in the original language, BTW.
Reaching for a foreign plural and getting the wrong one sounds awful, clunky, and also ignorant. But getting the right one often just sounds pretentious, and sometimes can even be misunderstood-- what if your audience doesn’t realize “apices” is the plural of “apex”? You really can’t go wrong with the plain -s when you are speaking English.
