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, Nurse Joy 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 stat screen. 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.
