what does pokerus do ???

Pokerus, although it sounds bad, is actually a good thing, and if you have it you are lucky.
Pokerus is a virus that randomly infects pokemon. While they have it, it doubles the EV's (effort values) that a Pokemon earns in a battle. Most people don't understand what EV's are though, so I'll put it another way. It makes the pokemon's stats raise higher/faster.
A Pokemon can only have the virus once. And it is spread to the other Pokemon on your team. It only lasts about 24 hours, so if you want to keep it around, infect a Pokemon with it and then put it in the pc. It will not go away while it is in the pc.

--Gamez_Freak--

Pokerus is awesome, lock that fact into your brain, so you can always remember it. Pokerus is a virus, or disease that Pokemon can catch randomly, but if your Pokemon gets it, then you are really lucky. Even though Pokerus is a virus, it does something good, while infected with Pokerus, your Pokemon will become even more stronger after each level up. That means get to work on leveling up, because Pokerus only lasts about a day. You can however, spread the Virus to your other fortunate Pokemon, so that they can level up really quickly as well. To Spread the virus, all you have to do is put the infected Pokemon in the PC, and the virus will eventually go into other pokemon.

Pokerus is a virus Pokemon get that is very good it raises the stats of your Pokemon can spread to other Pokemon in box and party if you keep in your party it will eventualy were off but in the Pokemon boxes it will last forever

the only bad thing is that it has a 1 in 8 chance of deleting your game

Pokerus' effects stay even when the virus is cured though