Have you ever been to the Pokemon Center and noticed that Nurse Joy is talking too much? If so, she may be talking about Pokerus. Pokerus is a virus that helps the EV training(doubles the amount of EVs you get per battle) of your Pokemon. When Nurse Joy is talking and says 'virus', you may think it is something bad. Well it isn't, it is actually something a skilled trainer wants. It can be even more helpful if you make your Pokemon hold the Macho Brace, quadrupling the amount of EV's per battle.
How do I get this 'PKRS'?
PokeRus is a random thing, so there is no sure way of getting it. You can spread it by putting the Pokemon in your Party. The virus will only spread for a certain time, so do it immeadiatly.
How long do I keep PokeRus
PokeRus is cured eventually, but it will show that your Pokemon had it once by have a smile by your Pokemon.
PokeRus is rare, but highly important, so hopefully one of your Pokemon get the virus.