Pokémon Ruby and Sapphire/Pokérus

The Pokérus is a virus that is beneficial to the growth of your Pokémon. Usually it is caught from a wild Pokémon. When you go to a Pokémon Center, the Nurse will explain about the virus:

"Your Pokémon may be infected with Pokérus. Little is known about the Pokérus except that they are microscopic life forms that attach to Pokémon. While infected, Pokémon are said to grow exceptionally well."

When placed in your party, it will infect other Pokémon in your party. When put in your PC the rate that it will disappear, and not infect any other Pokémon. Affected Pokémon have "pkRS" marked on their status screen.

Pokérus is cured after 24 hours on the party from the time of infection. After the virus wears off, a dot takes its place, meaning that Pokémon is now immune to the virus and can't catch it again. The accelerated stat growth remains, however; the only change is that the Pokémon can no longer spread it.

Pokémon cannot be cured of Pokérus if left in a PC box or traded to Pokémon Box: Ruby & Sapphire. This means you can deliberately infect one or more Pokémon you don't intend to use and then keep them in one of these two places, so you always have some infected Pokémon in your possession. The more infected Pokémon you store, the more backups you have if you accidentally leave one out long enough to be cured.