What is it?
Introduced in Generation II, Pokérus is a different kind of status condition. It can't be healed, neither through Pokémon Center nor a status healing item, such as the Full Heal or Lum Berry. However, each Pokémon can only get Pokérus once. Pokérus has two forms, active and cured. When a Pokémon has the active form, it can spread to the other Pokémon in the party. When a Pokémon has the "cured" form, it can no longer spread the virus to other Pokémon, however, it still gains all the other benefits from the Pokérus.
 What does it do?
Pokérus is a beneficial virus. Any Pokémon that has obtained it gains double the amount of Effort Values given during battles. This includes Power items, such as the Power Band and Power Anklet, but do not include Vitamins. For example: A Pokémon with Pokérus and holding the Power Weight that defeats a HootHoot will gain 10 HP EVs instead of the normal 5.
 How do I get it?
Pokérus is found rarely in wild Pokémon. To obtain it, the player must either defeat or capture the infected Pokémon. Finding a Pokémon with the virus is roughly the same chance of finding a shiny Pokémon, however the chance of catching Pokérus is rarer due to the wild Pokémon having no visible difference to tell the player it has Pokérus.
Once obtained, any Pokémon that has the active form can spread it to the other Pokémon in the player's party, though after a certain amount of time a Pokémon will be "cured" of the virus and be unable to spread it. To counter this, you merely have to stick a Pokémon with the active form into your PC. Any Pokémon with the active form are unable to be cured of the virus as long as they are in the PC. As Pokérus is very rare and very beneficial, it is recommended to infect a couple Pokémon and then deposit them into the PC so you have several Pokémon in the PC in case the one you're using to spread the virus accidentally becomes immune.