In return you can get a Poke of your choice (non-legendary) infected with Pokerus.
My dex is at about 500 across three generations, so I can provide almost any poke (I have all of the first 250 - i.e. not Celebi).
I'm craving a rain team and the 'Toed is the best choice obviously. And he's King of the Poliwags - how can you go wrong?