Pokérus

Pokérus is a virus in the game series that any Pokémon can catch if it has not already been infected once before. The effects of this virus are very beneficial to the Pokémon that receives it. Catching the virus however, is a very rare event, as the odds of meeting a wild Pokémon with it is 1 to 21,845 or roughly 4.57%. Similar to the Macho Brace, Pokérus will double the EV (effort values) stats of a Pokémon. A player can combine a Macho Brace and Pokérus to get a substantial amount of EV's.

When caught, the Nurse in a Pokémon Center will inform you if you have received it. The Pokérus is spread through training and battling Pokémon who do not have Pokérus with a Pokémon who does have it. The virus does not last forever, after two days or so it will wear off of the Pokémon and will not be able to be received again, however, if placed in the PC box, Pokerus will last until you take it out.there is 1/4 chance for a pokemon to catch the pokerus virus