I believe (but I could well be wrong): one virus, two or more virii
As the forum's official pedant, I must disabuse you of that grammatical horror.
The english word 'virus' is of course derived from the same latin word, meaning 'poison'. There is no known instance in latin of its ever having a plural, and latin scholars are in some disagreement about whether it was a noun of the second or fourth declension. If it was second declension, the plural would have been 'viri' (with
one i ) and if fourth declension the plural would have been 'virūs'. Or it might have been a word with no plural (e.g. a word like 'oxygen'). Nobody knows, but one thing is certain, and that is that the plural wasn't 'virii'.
In English, the plural is 'viruses'.