http://en.wikipedia.org/wiki/Candiru
Historical accounts
The earliest published report on this candiru attacking human hosts comes from German biologist C. F. P. von Martius in 1829, who never actually observed it, but rather was told about it by the native people of the area, including that men would tie a ligature around their penis while going into the river to prevent this from happening. Other sources also suggest that other tribes in the area used various forms of protective coverings for their genitals while bathing, though it was also suggested that these were to prevent bites from piranha. Martius also speculated that the fish were attracted by the "odor" of urine.[7] Later experimental evidence showed this to be false, as the fish actually hunt by sight and have no attraction to urine at all.[8]