The Pokerus is a virus that is beneficial to the growth of your Pokemon. Usually it is caught from a wild Pokemon. When you go to a Pokemon Center, Nurse Joy will explain about the virus:
"Your Pokemon may be infected with Pokerus. Little is known about the Pokerus except that they are microscopic life forms that attach to Pokemon. While infected Pokemon are said to grow exceptionally well."
When placed in your party it will infect other Pokemon in your party. When put in your PC the rate that it will disappear, and not infect any other Pokemon. 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.
