I obtained a virus called PokéRus (PKRS) what does it do?
PKRS helps with EV training. When you go to heal your Pokemon at the Pokecenter Nurse Joy tells you it's a virus and will go away eventually. Having PKRS though, is a good thing. It doubles the EVs you get from a battle. It works basically the same as a Macho Brace, but you don't have to have a hold item, it just happens. You can hold items such as Macho Brace and have PKRS at the same time to make EV training even faster, but I'll explain that later.