It's this disease that Pokémon get at random that is actually a good thing, because it boosts their stats. Once someone has it, you can put it in a party with other Pokémon, and it'll infect them. It'll go away after a while, but the stat boost will stay. You can tell something has Pokérus by going to a Center and talking to the nurse, or just looking at it's picture, since it'll say POKÉRUS. It's really rare to get infected in the first place though.