Link: https://en.wikipedia.org/wiki/Candiru_(fish)

A long-standing claim about candiru is their supposed propensity to insert themselves into human urethras or other orifices in an apparent attempt to parasitize; this alleged ability has been described in ethnological reports dating back to the 19th century.However, these accounts of human parasitism are often biased, arising from "imprecise, second- and third-hand accounts, misconceptions, and folk tales"; which lead to the spread of imprecise or dubious claims that may even be reported in scientific resources; there are no credible reports of candiru parasitizing human urethras.

A naughty little fish with a penchant for swimming up a man's urethra!