Pokérus

Pokérus is a fictional "disease" in the Pokémon series of games. It has two stages. The first stage has a chance of spreading to other Pokémon in the player's party. After a while, the Pokérus moves to its second stage (i.e. is overcome). In this form, Pokérus does not spread to other Pokémon.

Pokérus has the effect of doubling EV gain for affected Pokémon.