Yes, to be more exact, it WAS infected by a virus called Pokerus that doubles the EVs obtained from a battle. This Pokemon you got is unable to spread Pokerus or get infected by it again, although it still gets double EVs from a battle. That's why you should always keep a couple of Pokemon, infected by the Pokerus, in the Boxes since they can't get cured in there. If you need more information, just search for other topics similar to this.