The Pokemon games (starting with generation II) feature an in-game virus that Pokemon can catch called Pokerus. It can't be healed at a Pokemon center or with items, and can only be contracted by the Pokemon in question once. It has a 3 in 65,536 chance of generating on a Pokemon, either wild or bred. The virus causes Pokemon to gain double the effort values from battles.

The virus will eventually dissipate after two days, all the while remaining highly contagious, though placing an infected Pokemon in the PC will cause the virus to last indefinitely. After the virus is gone, a smiley face (a red dot in Generation II) will appear on the formerly infected Pokemon's status screen, and the positive effects will still remain, particularly the increase in effort values