Red belly Piranhas can be aggressive towards humans and if they feel threatened they can attack humans.
But some people have emerged safely from the waters when swimming with Red Belly Piranhas so although they can be dangerous they don't always bite people or eat the flesh off of humans.
But still be very cautious and avoid any waters with any type of Piranhas including Red Belly Piranhas because there is always the chance they could decide to attack a human at any given moment.