These fish go up your pee pee hole!

When the candirú successfully invades a human, it proceeds exactly as it would with a fish host. After entering the misidentified orifice, it quickly wriggles its way in as far as possible, often accompanied by the victim's frantic attempts to grip the slippery, mucus-coated tail. In the unlikely event that the panicked victim manages to grasp the fish, its backwards-pointing barbs would cause excruciating pain at each pull, and bring a quick end to the dramatic tug-of-war. Once inside, the parasite inches its way up the urethra to the nearest blood-gorged membrane, extends its spines into the surrounding tissue, and starts feasting.

Damn Interesting » The Terrifying Toothpick Fish