Modern linguists view language as a system of rules and processes that generate forms, phonological, lexical, morphological, semantic. A word ''belongs'' to a language by virtue of its participation in the network of those rules. There is no such thing as a Latin, English, Chinese, Hindi word that exists apart from its participation in the rules of a natural language. That being the case, it is natural that the form virus, when borrowed into English, should conform to English rules of pronunication and to English rules of plural formation.- from Linguist list, a discussion that goes far beyond the initial question:
The view of this Latin scholar is that ''virus'' has no attested plural in Latin. It was an unusual, rather rare, indeclinable mass noun. I don't know the actual origin, but most Latin speakers probably thought it was Greek or some other neighboring language.
Latin / English plural of 'virus'