Pokémon Gold and Silver/Pokérus

Pokérus is a rare disease that occurs 1 in about every 32000 on a wild Pokemon. Now don't go running off every time you see a Pokémon with the Pokérus, it is actually helpful: the Pokémon gets double exp. When you get it, professor Elm will ring you, he will say that one of your Pokémon has Pokérus, a Pokémon virus and something along the lines of "it will go away after a few days".

(In 3rd Geeneration Games, you get double EVs.)