Candiru Fish

Although lurid anecdotes of attacks on humans abound, there is only one documented case of a candiru entering a human orifice. In this instance, the victim had a candiru swim into his urethra as he urinated while thigh-deep in a river. Jeremy Wade, a British biologist, investigated this incident for the Discovery Channel's River Monsters. The victim underwent a two-hour urological surgery to remove the candiru. Dr. D. Scott Smith, Chief of Infectious Diseases at Kaiser Permanente, described the candiru as having spikes to assist it clinging to its host.
It was also once thought that the fish was attracted to urine, as the candiru's primary prey emits urea from its gills, but this was later discredited in formal experimentation.
"The candiru usually targets a big fish as its host, drinking blood from its gills, but occasionally they make mistakes. This has resulted in one of the most infamous legends to emerge from the Amazon: a man urinating in the river who has a fish swim up his penis." – Jeremy Wade, biologist.