Expansion on the concept:
You wish to know why? When ever you pluralize a word that ends in a long vowel sound, such as 'antenna', the ending is changed to a long 'e' sound ('eee'). So, antenna becomes antennae, and virus becomes viri (exception to the case are words like banana and cacti. It's pronounced 'cac-aye'. There are specific cases when the end is pronounced 'eee', and what these cases are, I do not know).
Now, take radius... you have an 'i' that you must pronounce; ray-dee-us
The plural form should be, according to the rule, 'ray-dee'; however, there is still that extra 'i', thus, it becomes 'ray-dee-aye'
With virus, there is no extra 'i', thus, instead of being pronounced 've-ree-aye', it's pronounced 've-ree'.