I'm holding this word responsible for all kinds of ludicrous word-mangling in the realm of 'virii,' 'Jesii,' and anything else ending in -ii.

Look at it again: the plural form of radius is radii. Did you see what happened there? A Latin word ending in -us becomes plural by turning the 'us' into 'i'. Since 'radius' has another 'i' before its 'us,' we get 'ii.'

'Virii' would be a plural form of a regular Latin word Virius, which afaik, does not exist.

And no, the plural of 'bus' is not 'bi,' and it's not even irregular. Words ending in 's' are made plural by adding '-es', hence 'buses.' (Even though it looks like 'byooses'.) Also note that this is a general rule, broken by countless exceptions such as the above Latin '-us -> -i' and '-is -> -es'. Remember, you can never learn English; you can only hope to memorize it.