Bigfrog Weblog

Apparently, there's another good reason not to go skinny dipping in the Amazon ... and it's not because of the piranha:

[The candirú] 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 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 surgery. 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 honing in on a human urethra.

See Candirú in Wikipedia for all the gruesome details.