Pokémon Gold and Silver Versions/New Features/Pokérus

Pokérus (a portmanteau of "Pokémon" and "virus") was introduced. Encountering a Pokémon with the virus is even rarer than encountering a Shiny Pokémon, with the odds estimated to be somewhere around a 1 in 21,845 chance. Pokérus doubles the special experience (a concept adapted to later installments as effort values) that the player's Pokémon gain each time the infected Pokémon participates in battle (provided the battle is won and the infected Pokémon does not faint).