Fictitious plural of the fictitious Latin word virius.
My ISP used to have "Virii are real" in big fat letters on its home page. I sent the owner an email saying, "No, they're not, but viruses are." He fixed it immediately. Must have felt quite embarassed, since it is not the type of error he was likely to make. I suspect he must have been answering a tech support call while he was typing the original, so he was distracted.
Ironically, if the word virius actually existed in Latin, it would mean something like manlier, which might indicate that virii is a "macho" mistake. But it doesn't, so it isn't.