Pokémon Gold Version, Silver Version, and Crystal Version - Pokérus
Pokérus is a virus that infects Pokémon. There is a 3/65536 chance that a Pokémon will become infected with Pokérus at the end of a battle in which Exp. Points can be gained.. (Even Eggs can be infected this way.) A Pokémon infected with or cured of Pokérus will earn twice as much stat experience as normal in battle.
For infected Pokémon, the status line reads Pokérus, even if the Pokémon is poisoned, burned, paralyzed, frozen, or asleep. When the player tries to heal the infected Pokémon in a Pokémon Center, the receptionist will notify the player of Pokérus.
Whenever a battle ends, for each Pokémon and Egg infected with Pokérus, there is a 1/3 chance that both the Pokémon that comes before and the one that comes after that Pokémon are also infected with Pokérus if not already infected. (Even Eggs can be infected this way. The game checks whether this condition will apply for each Pokémon already infected before infecting others if necessary.) Walking with an infected Pokémon in the party or placing an infected Pokémon in a PC storage box won’t infect other Pokémon.
Within four days, at midnight, the Pokémon is cured of Pokérus. Once a Pokémon is cured of Pokérus, it will never get it again (in the stats screen of such a Pokémon, a black dot will appear). However, a Pokémon kept in a PC storage box will remain infected.