Fish name: Piranha
Origin location: South America
US locations: Various lakes, including Winnebago in Wisconsin, Ozarks in Missouri, St. Clair in Michigan, Tom Bass Park in Houston.
Why they're dangerous: Teeth, teeth, teeth! The piranha's teeth are incredibly sharp, and they can bite hard.
Should I be concerned?: As a swimmer, no, not necessarily. Piranha's are not likely to bite humans--its actually safe to swim with piranha given that they have other food, or they aren't hungry.
As a fisherman, though, they can be dangerous. If you accidentally hook a piranha in one of their US locations, getting it back off your hook can be dangerous because of their teeth. To be honest, it's not a huge concern because they're more likely to nibble your fresh bait away than they are to be hooked. In fact, in South America, many people try to catch piranha and fry and eat them like other fish.
The bigger concern when it comes to piranha is that they can eat other fish, which might (long term) hurt the fish population of the areas you are in.