Is puffer fish even sold in stores? I thought it was one of those specialty 'russian roulette' dishes in restaurants.
TTX (tetradotoxin) is a lethal neurotoxin produced by puffer fish, its a Sodium channel blocker which essentially prevents the muscle cells from contracting, thus paralysing the diapraghm which helps your lungs 'breathe'.

You don't cook fugu, ie pufferfish. And it can be very dangerous to eat it raw.