Pokémon Gold & Silver
PokeRus

Even though it is extremely rare, it is possible for your Pokémon in Pokémon Gold/Silver to catch a virus. The chance to encounter the virus is extremely slim so you may consider yourself "lucky" if your Pokémon is infected by the virus. You may not notice the infection at first, but when you bring an infected Pokémon to a Pokémon Center, Nurse Joy will tell you that there are small life forms stuck on the Pokémon. She doesn't know much about the virus but she insures you that your Pokémon is fine. You will then get a phone call from Professor Elm saying that the PokéRus is harmless, has no apparent effect, and will wear off in time. Since the Pokerus is a virus, it does spread itself among other Pokémon it contacts. If you put an infected Pokémon into a PC Box, other Pokémon in the box will also be infected. Carry an infected Pokémon with you will also cause your other Pokémon you are carrying to be infected. Once a Pokémon is infected, it will take approximately five days before it is recovered. A recovered Pokémon will become immune to the Pokerus. This is indicated by a small dot that appears on the Status screen. Pokerus is really harmless (actually, helpful), while your Pokémon is infected or after it is cured, the Stats of your Pokémon will increase twice as much during battles.

You can tell your Pokémon is infected by PokeRus by viewing the status of the Pokémon. An infected Pokémon will show PokeRus under its status. If this virus is so precious, is there other way to get your Pokémon infected by it??? Yes, you can trade an infected Pokémon to your friend, or you may use Gameshark. The codes are Poke 1 010146DA, Poke 2 010176DA, Poke 3 0101A6DA, Poke 4 0101D6DA, Poke 5 010106DB, and Poke 6 010136DB.