Tropical Freshwater Fish
Piranhas are predators living in shoal, feeding mainly on fish, preferentially the sick or injured ones. In this way they prevent spreading of plague – they are something like health police.
There is no danger for humans to swim in areas where piranhas live, as there has never been an evidence of an attack or even death caused by them. A bite is all what can happen.
Indians fish piranhas for food and use their strong and sharp teeth as scissors or razor.