First, I said PREFERRED, not obligatory. It is tolerated to say 'viruses', but 'virii' is considered better usage, as I've been taught in college. Don't blame me, I didn't create the English language.
If it is of any interest, in French it is considered good usage to use the French plural for foreign words. As such, we say << des spaghettis >>, while 'virus' as in << des virus >> is invariable. But since the main language of this Forum is English, it is best to follow the rules of the English language.