Pokérus

In other words, it's actually really good to have Pokérus.

Other things you should know about Pokérus:
-It will go away after a day or two.
-Once it goes away your Pokémon cannot get it again.
-Pokémon that are cured from Pokérus will still get double EVs, but cannot spread the virus.
-Pokémon that are cured from Pokérus will have a red dot appear by their HP bar (Gen 2).
-Pokérus can be spread to other Pokémon in your party.
-A Pokémon infected with Pokérus that is kept in the PC will not lose it.
-If you leave a Pokémon infected with Pokérus in your party and turn off the game it is more than likely that it will be cured by the next day. I.E. leave that Pokémon in your PC!