PKRS has been rumoured since Red/Blue days (I recall a lot of the "cheats" having to do with the E4, et cetera, but they all proved false). I've never actually gotten it until Ruby/Sapphire and even then I used a gameshark to see if it was real or not (so I didn't find it in-game).
Drew: PKRS (pokerus) is a status condition that spreads from one pokemon to the rest of your party and dissapears in a few days. I think the effect is that it doubles EVs. You can keep the status by putting an infected pokemon in the PC.