The latin plural of 'virus' does not exist. It is a nebulous, uncountable concept to ancient rome. Like 'malaise' or 'ocean' (we can pluralize oceans in english, but hopefully you can imagine that a language would treat it as a concept for which the concept of pluralization doesn't make sense).
You'd think: Hey, it's `-us`, I know that one, it pluralizes to `-i`, but, no. There is no plural. It's a rare form that also ends in -us but doesn't work like most of the words you know derived from latin that end in -us.
But there is `vir`, latin for 'man', and the plural of that is viri.
So, 'viri', as in 'the latin pluralized form of virus', is dead wrong.
There is no -ii ending in latin. At all.
The plural of 'virus' in english is 'viruses'. Any other form is stupid. Sometimes language does that, but surely you'd agree that something like 'irregardless', whilst somewhat common, sounds very unprofessional. virii is the same way.