Pokemon Gold and Silver - PokéRus virus

Sometimes, after a battle, one of your Pokémon will be infected by a virus called PokéRus. There is a 3 in 65536 chance that this will happen, even slimmer than finding a shiny Pokémon, so you probably won't have the chance to feel this effect.

Infected Pokémon have a status that reads PokéRus. When you try to heal the infected Pokémon in a Pokémon Center, the receptionist will say:

Your Pokémon appear to have tiny life forms stuck to them. Your Pokémon are healthy and seem to be fine. But we can't tell you anything more at a Pokémon Center.

Then, Professor Elm will call:

I discovered an odd thing. Apparently there's something called PokéRus that infects Pokémon. Yes, it's like a virus, so it's called PokéRus. It multiplies fast and infects other Pokémon too. But that's all. It doesn't seem to do anything, and it goes away over time. I guess it's nothing to worry about. Bye!

When the infected Pokémon is placed in a Box in Bill's PC, all the Pokémon in the box may be infected. Also, walking around with the infected Pokémon or healing your infected Pokémon at a Pokémon Center may make all Pokémon in your party infected (about 1/3 chance). It disappears within four days at 00:00 hours. Pokemon kept in a box, however, will remain infected. After a Pokemon is cured of the virus, it will never get it again (a black dot will appear in the summary screen of such Pokemon).

What the PokéRus does is double the Stat Exp received during battles, even when the Pokémon is cured of the virus.

Portions thanks to Meowth346.