Pokerus is a virus your Pokemon can contract. There's only a very small chance of it happening, but if it does the stat gaining of that Pokemon will be significantly larger than before. Also, if I recall, it can spread to the other pokemon in your team/in the box that it is in. The virus isn't permanent, however, and it will go away eventually. The best way to counteract this is to stick an infected pokemon in a box, as the pokerus does not leave the pokemon if it's in one.