I know that virii may look correct, but 'viruses' is the correct plural for virus. For 'virii' to be correct (wandering into Latin land with this), the singular would have to be 'virius' . In fact, the plurals for words ending in 'us' is a bit of a morass. Octopus/octopi, radius/radii, campus/campuses, anus/anuses, (campi? ani?), and one to throw a spanner into the works, genus/genera.