I'm glad cause my Pokémon caught the Pokérus while building up stat experience in the Ruins of Alph
Anyway, here's some findings on the Pokérus...
At the time I got infected with it, I had an egg in my party. All the Pokémon in my team had the Pokérus and when my egg hatched, I looked at it right away. (I might have taken a few steps, but... it had the Pokérus!)
So, the egg was in my party when the original strain of PokéRus infected my team. When it hatched, it was born with Pokérus. The parents of the egg did NOT have Pokérus (just so ya'll don't blame it on that, the egg's parents couldn't had it cause I didn't have PokéRus yet...)
Anyway, the PokéRus is transferable from Pokémon to Pokémon I'm just not sure how. I caught a Magikarp & nicknamed it Disease. After some time in my party, it caught the Pokérus after a battle. Maybe it was from the battle, maybe enough steps in my party, I dunno. I caught another Magikarp and tried to infect this one, but, I couldn't make the new karp get infected and my team still all had the virus, so, I don't know for sure WHAT transfers the virus.
Anyway, I hope my findings help you out somehow.