Sharks kill several people every year, probably because they mistake swimmers for seals, which are often their normal prey.
The small piranha can also be extremely dangerous. Piranhas live in rivers in South America. They are stocky disc-shaped fish, about 30 cm in length, with a blunt head. Piranhas feed by biting out a crescent-shaped piece from their prey with razor-sharp interlocking teeth. They can give a nasty bite, but are not really dangerous unless they attack in large numbers. Piranhas are supposed to be able to strip all the flesh from a pig or cow in a few minutes, but they are probably not dangerous to humans unless they are attracted by blood.
Picture credit: google