Pokérus is an ultra rare disease that occurs 3 in every 65,536 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.
Tips
actually pokerus does NOT double exp. It doubles the EV's gained from battling wild pokemon. EV stands for effort value. Effort Values eventually increase specific stats, depending on the type of pokemon battled. Essentially, if you want your pokemon to have max health, you battle pokemon that give EV's in health, and if your pokemon has pokerus, it halves the number of battles you'll need to max your EV's. As for transferring pokerus, the pokemon you desire to be infected must be bordering another pokemon in your party who already has the pokerus. In addition, it may not be bordering pokemon who have had the virus, but are now cured.
Cured Pokémon
A cured Pokémon will still get double EV's, but will no longer be able to transmit the pokerus to other pokemon.
Preserving Pokérus
To keep the pokerus forever, simply store a pokemon with the virus in your box and it will never be cured as long as it remains in the box! It is best to give the pokerus to multiple pokemon just for storage. That way you always have it!