What??

Pokerus is a thing your Pokemon get (kinda like a good virus) that, upon leveling, boosts the stats by a lot. Pokerus spreads in the party by double battling. Unfortunatley, it doesn't stay forever so when it leaves the Pokemon, it can't get it again. So get all the party Pokemon (the 6 Pokemon with you) and just level up as quickly as you can!

Pokerus is a GOOD virus that helps your Pokemon grow stronger if they are holding a Macho Brace. You infected Pokemon can transfer it to another Pokemon if they are next to each other in your party.

Its a cold they get, it helps to improve thier stats when the grow. It's a good thing for the pokemon. It will spread to the rest of the Pokemon in your party( the ones you carry with you). Also this only last a few days, then the Pokemon will have a smiley face and not be able to get it anymore. How ever if you put a Pokemon in the computer with the virus it keeps the virus. So I keep extra Pokemon with the virus in the computer till I need to give it to other Pokemon when I catch a few later on. So keep many with the virus in the computer till you need to use one. Remember to have the Pokemon with the virus as the first Pokemon to spead it to the other Pokemon faster. An once you bring the Pokemon from the computer just use that one and keep the rest that has the virus for a later time in the computer. Any Pokemon that you just infested with the virus you can put in the computer right then an level them up later.

Basically, there is something called Effort Values (EV) in the game. By fighting a wild Pokemon, the Pokemon that fights it earns 1 EV. The type of the EV it benefits (HP, Attack, Defense, Sp. Attack, Sp. Defense, Speed) depends on the wild pokemon. By getting 4 EV, the stat will go up by one point the next time it levels up. The PokeRus cuts that time in half, however, because if your Pokemon has the PokeRus and fights a wild Pokemon, it will earn 2 EV from fighting the wild pokemon. If your Pokemon has the PokeRus and is holding a macho-brace when you fight a wild Pokemon, you will earn 4 EV, automatically giving you 1 stat up. This is the most efficient way of jacking up your pokemon. Here is a list of the Pokemon that help in specific stat raising techniques:

HP-Shellos/Bidoof-Valley/Windworks
Attack-Machoke/Bibarel-Route 211 East
Defense-Graveler/Onix-Iron MT
SP.Attack-Gastly/Haunter-Old Chateau
SP.Defense-Tentacool/Tentacruel-anywhere you surf
Speed-Staravia/Pikachu-Trophy Garden

These Pokemon are relatively easy to find, and with the PokeRus and macho brace, your Pokemon will gain status points extremely quickly. Hope this helps!!