There can be few species on Earth about which more myths exist than piranhas.
Piranhas have an aggressive image as flesh-eaters, able to dilacerate a human body in seconds. While piranhas do occasionally attack humans, this is only usually when water levels are low and there has never been an official recorded death due to piranha attack. It is true that many humans have lost a finger or toe to piranhas, but death is highly unlikely, despite many people swimming in piranha-infested waters in South America on a regular basis. The myths about piranhas attacking humans concern the red-billed piranha, Serrasalmus nattereri, which is not the species shown in the image. It is widely believed that these piranhas can be attracted by blood and are exclusive carnivores, neither of which are true. In Brazil, it is also believed that piranhas sweep rivers at high speed and attack cattle entering the water. This is also untrue.
In reality, even of the most vicious, carnivorous piranhas only tend to attack animals smaller than themselves, such as other piranhas, fish, frogs, insects or birds. It is only if they are extremely hungry, or if the victim is weak or injured, that they will attack larger animals. Finally, with regards to the large schools it is thought piranhas are supposed to hunt in, it is now known that this is actually a defence mechanism against natural predators such as dolphins, giant pirarucu and caimans (which, incidentally, I would be much more concerned about if I were to take a dip in the Amazon).
Interestingly, it seems that many of the myths surrounding piranhas can actually be traced back to former U.S. President Theodore Roosevelt’s 1914 book, Through the Brazilian Wilderness:
“They are the most ferocious fish in the world. Even the most formidable fish, the sharks or the barracudas, usually attack things smaller than themselves. But the piranhas habitually attack things much larger than themselves. They will snap a finger off a hand incautiously trailed in the water; they mutilate swimmers—in every river town in Paraguay there are men who have been thus mutilated; they will rend and devour alive any wounded man or beast; for blood in the water excites them to madness. They will tear wounded wild fowl to pieces; and bite off the tails of big fish as they grow exhausted when fighting after being hooked.”
“But the piranha is a short, deep-bodied fish, with a blunt face and a heavily undershot or projecting lower jaw which gapes widely. The razor-edged teeth are wedge-shaped like a shark’s, and the jaw muscles possess great power. The rabid, furious snaps drive the teeth through flesh and bone. The head with its short muzzle, staring malignant eyes, and gaping, cruelly armed jaws, is the embodiment of evil ferocity; and the actions of the fish exactly match its looks.”
“I never witnessed an exhibition of such impotent, savage fury as was shown by the piranhas as they flapped on deck. When fresh from the water and thrown on the boards they uttered an extraordinary squealing sound. As they flapped about they bit with vicious eagerness at whatever presented itself. One of them flapped into a cloth and seized it with a bulldog grip. Another grasped one of its fellows; another snapped at a piece of wood, and left the teeth-marks deep therein. They are the pests of the waters, and it is necessary to be exceedingly cautious about either swimming or wading where they are found.”
“If cattle are driven into, or of their own accord enter, the water, they are commonly not molested; but if by chance some unusually big or ferocious specimen of these fearsome fishes does bite an animal—taking off part of an ear, or perhaps of a teat from the udder of a cow—the blood brings up every member of the ravenous throng which is anywhere near, and unless the attacked animal can immediately make its escape from the water it is devoured alive.”
You can find advice on how to swim safely with piranhas at this hilarious wikihow blog here.