The candiru fish is a species of parasitic freshwater catfish. It normally attacks the gills of large fish. 

Supposedly, it can swim up a stream of urine and into a person’s urethra if they pee in a river that the fish lives in. This is not physically possible, both because you can’t swim up a falling stream of liquid and because the fish is too large to fit into the urethra. Babies are not too large, but have no incentive to swim up someone’s urethra.

Supposedly this is because they’re attracted to the scent of urine. They are not, they hunt by sight. To be fair, this was only discovered recently.

Supposedly the people who live in candiru habitat wear coverings to keep the fish out of their junk. Those coverings are for piranha defense. 

Supposedly there are people who’ve had to have their penis amputated after a fish swam up it. Again, piranha, in this case responsible for the amputation. 

There have been a few cases of an apparently confused candiru fish ending up in someone’s vagina when they were swimming naked. 

So, basically, don’t swim naked in the Amazon. If you have a vagina, the chance of getting a fish up it is very small, but you still don’t want to deal with things like piranha. Also, minnows nibbling on your junk probably doesn’t feel good.