Pokémon Gold and Silver/Pokérus

Pokérus is an ultra rare disease that occurs 1 in about every 32000 on a wild Pokémon. Pokérus is helpful. It will double the exp of the infected Pokémon. After a few days the Pokémon will heal automatically, and will have a small dot on his stat page.

Cured Pokémon
A cured Pokémon will still get double exp, but cannot infect others.

Preserving Pokérus
In order to prevent Pokérus from stop spreading, store one infected Pokémon in the PC and he will stay infected forever. (PC stored Pokémon do not transfer Pokérus, but remain infected forever). When you want another infected Pokémon, just take out the infected Pokémon from the PC and put it in your party. The rest of the party will quickly be infected.