[DrJokepu]

Because with [radi]us the stem is "radi" but with [vir]us the stem is "vir". These words are from the same type (second declension) they both receive an -i affix in plural, hence radi + i = radii, vir + i = viri. Latin being Latin there are an awful number of exceptions but this is a somewhat general rule.

Put it like this: Grammatically speaking, the plural of virus is viri. Putting it into plural might or might not makes sense. Personally, I don't think that using plural for collections in Latin is a very big sin given that this is very common in classical Latin texts.

One example of this can be found in the famous introduction of Aeneid (I.1 "Arma virumque cano...") lines 31-32, where Virgil is using the plural form of the word "sea" (mare, plural: maria)

"multosque per annos / errabant acti fatis __maria__ omnia circum" - "for a number of years, driven by fate, wandering around on the seas"