From Bulbapedia, the community-driven Pokémon encyclopedia.
The PokéRus is a Pokémon enhancing virus. It first appeared in Pokémon Gold and Silver.
In that game, Pokémon had a 1 in 21,845 chance to become infected with the virus. This slim chance meant that most players would never encounter it.
Pokémon also obtained from a Pokémon Rocks America event organized by Nintendo came infected with the virus.
This disease is actually a good thing. After being infected, a Pokémon's status will read "PokéRus". A nurse in a Pokémon Center will notice that there are "small life forms" on the Pokémon. Professor Elm will then call the player to say that the PokéRus has no effect and will wear off.
A Pokémon with the PokéRus will be able to infect other Pokémon if it's stored in the PC with other Pokémon. It can also infect other Pokémon in the Trainer's active party.
Healing a PokéRus infected Pokémon at a Pokémon Center will also infect every other Pokémon in the party.
In Pokémon Gold and Silver, it took about five days for a Pokémon's immune system to fight off the virus. After recovering from the virus, a little dot appears on the Pokémon's status screen indicating that it is immune to the virus and impossible to reinfect.
The PokéRus doubles the Effort Values received from battles. This causes the stats of a Pokémon to grow twice as fast while still leveling up at a normal rate. This is especially helpful in raising Pokémon caught at a high level, since it is difficult to raise their Effort Values to maximum before Level 100.
After a Pokémon's immune system fights off the virus, it will still be able to gain double Effort Values permanently. However, it will not be able to infect other Pokémon with the beneficial enhancement at that stage.