What is Pokerus?

Pokerus, is a rather nice virus that your Pokemon can catch, that can make them 'sick' though how sick, I cannot say. Even though it is a Virus, it is helpful for the Pokemon, because while they are infected with Pokerus, they will grow levels faster than they normally would. And the best part of the Virus, is that it is Contagious, so you can spread it to other pokemon!

Like cheating-is-good said, it is a virus that randomly affects your pokemon.
While they have it, they're stats will raise more than normal each time your Pokemon levels up. Also, like he said, it is contagious, but it is temporary, lasting only about 24 hrs. And once a Pokemon gets it, it can't get it again, so use it wisely. There is a way to keep it around, though. Take a Pokemon that has it and store it in the pc. Although it won't spread, it won't go away for some time after you take it out of the pc. Hope I helped.

--Gamez_Freak--

Pokerus gets caught randomly when you give your Pokemon to the Pokemon center nurse to be fully healed. The best thing to do with Pokerus is to have a nice battling and levelling frenzy with your infected Pokemon, because they will grow much faster. Try to heal their HP and PP an status with medicines NOT the Pokemon center because the Pokemon center can destroy Pokerus.

The pokerus is a virus that Pokemon catch randomly. It helps because it makes Pokemon lvls grow faster and their stats get higher. This is a contagious virus too. But make sure you always have at least 1 Pokemon have it in a box and almost never take it out because Pokemon can lose the virus and once they have, they are immune to it