People are welcome to form a fake Latin plural of "virus" if they wish - I just wondered where that extra "i" came from.
"Opera" is itself a plural - it's the plural of "opus" ("work", in the sense of an artistic work)
I don't object in the slightest to English plurals. Once a word is used in English, it is an English word, and can legitimately be given an English plural. Thus, "viruses", "focuses", "appendixes", and so on, are all absolutely fine.
What I think is a little bit silly is to "make" up Latin plurals - and even worse, to get them wrong
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I had a rant about Italian plurals in another thread recently. "Paninis" is one that's often seen - people genuinely don't seem to be aware that "panini" is a plural.