pokerus, i hasten to add, is NOT A BAD THING. you should actually keep one of your infected pokemon in the PC so you can pass it on to other pokes you plan to raise.
what it does, in simple terms, is that it makes pokemon gain stats faster when they're in lower levels. as you get toward higher levels, the difference tends to even out, but it also makes EV training time a lot shorter, if you know what that is.