The candirú or canero (Vandellia cirrhosa) or toothpick fish is a freshwater fish in the group commonly called the catfishes. It is found in the Amazon River and has a reputation among the natives as the most feared fish in its waters, even over the piranha. The species grows only to a size of an inch in length and is eel shaped and translucent, making it almost impossible to see in the water. The candiru is a parasite. It swims into the gill cavities of other fishes, erects a spine to hold itself in place, and feeds on the blood in the gills, earning it a nickname as the "vampire fish of Brazil".
It is feared by the natives because it is attracted to urine or blood, and if the bather is nude it will swim into an orifice (the anus or vagina, or even in the case of smaller specimens the penis - and perhaps deep into the urethra). It then erects its spine and begins to feed on the blood and body tissue just as it would from the gills of a fish. The candiru is then almost impossible to remove except through an operation. As the fish locates its host by following the water flow from the gills to its source, urinating while bathing increases the chance of a candiru "homing in" on a human urethra.
So remember amigos. When in the amazon river. Never ever pee and swim simultaneously.