Pokérus
Pokérus is a rare, harmless virus that doubles the effort values a Pokémon earns while infected
Gold and Silver introduced Pokérus. A Pokémon has about a 3 in 65,536 chance of carrying it when encountered. The virus spreads to adjacent party members and cures itself after 1 to 4 days, but the doubling stays for good.
Main-series games; introduced in Generation II, Gold and Silver.
One later change
- Ruby and Sapphire applied the doubling to EVs instead of stat experience.