Pokerus is a virus that infects your Pokemon. But instead of being negative, it is actually very good. I'm not entirely sure how it works, but it helps the stats of your Pokemon infected with it, I think.
As for the advantage, your Pokemon (let's just say a Magikarp) would be more powerful than another Magikarp of the same level.
Pokerus spreads, but I don't remember the condition. As for finding out, I'll link the same way DNA did:
Psypoke Pokerus
Sorry if the link doesn't work, I'll see what I can do if it doesn't.
AEPMT
