Pokérus

Pokérus is a Pokémon virus. It is a condition you get from wild battles. Don't be alarmed if your Pokémon has this virus. In fact, it is actually good for your Pokémon. Pokérus gives you double effort value points. It can spread through your party, but not in your PC, though you don't lose it. Pokérus can be handy when you want to train some of your weak Pokémon.