I honestly have no idea how I got Pokerus in Diamond. I didn't even know I had it until the pokemon center nurse mentioned it, and then my entire team caught it. Still had no idea what it was until I started playing Black 2, noticed the medal for Pokerus Discoverer, and looked it up. After discovering that I checked my PC in Diamond and found no less than four Pokemon who had it and were still contagious. Sent one, a Manectric, over to Black 2 and let my team catch it. While they were still contagious I did a buttload of breeding and grind-evolving, so now half the pokemon in my PC have it.
When I started White 2, as soon as I was able to get into the union room I sent a pokerus-positive riolu from Black 2 to White 2. So all my pokemon over there caught it early, and they're all going to be beasts when they get into the higher levels.