Pokémon Ruby Version, Sapphire Version, and Emerald 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 many base stat points (EVs) as normal in battle (see the "Base Stats (EVs)" section).
Whenever a battle ends, for each party 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.
A Pokémon with Pokérus will remain infected from 1 to 4 days. After that time, at midnight, the Pokémon, if it’s in the party, is cured of Pokérus. Once a Pokémon is cured of Pokérus, it will never get it again (in the summary screen of such a Pokémon, a black dot will appear). However, a Pokémon kept in a PC storage box will remain infected; such a Pokémon’s infection won’t progress as days pass.
Pokémon FireRed Version and Pokémon LeafGreen Version don’t have access to a real-time clock, thus in these versions, a Pokémon can’t be cured of Pokérus and a Pokémon can’t become infected with it in any way. However, a Pokémon already with Pokérus can be traded to these versions and will still be under the effects of Pokérus in these versions.