Most species are carnivorous, but not all, some using their serrated teeth to eat fruit. A school of the red piranha can skeletonise a cow in under two minutes  . Consequently, they are one of the most feared fishes of South America, along with the electric eel and the candiru.
Piranha's voracious appetites make them easy to fish for (though hooked fish will be devoured by other members of the school unless reeled in swiftly); their white flesh, though filled with many small bones, can be a valuable source of protein to inhabitants of the Amazon River region.
There are no documented human deaths from piranha attacks. 
- Encarta encyclopedia Piranha