http://en.wikipedia.org/wiki/Candir%C3%BA

It is feared by the natives because it is attracted to urine,[3] and if the bather is nude it will swim into an orifice (the vagina and even the penis—and deep into the urethra) and because of spines protruding from the fish, it is almost impossible to remove except through surgery.[4] The fish locates its host by following a water flow to its source and thus urinating while bathing increases the chance of a candirú homing in on a human urethra. Natives have also been known to bathe facing the current, as doing so would decrease the chances of the organism lodging itself in the rectum.[citation needed] Other orifices such as the penis or vagina are covered up with the use of hands.

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