The Parasitic Candirú

From DamnedInteresting.com, comes the story of a parasitic vertebrate, a fish known as the toothpick fish or Candirú, that is attracted to the ammonia released from the gills of fish as they breathe. That in itself would not be very bad at all, except for that fact that they can't distinguish between than and a human relieving himself in the Amazon. The fish swims up the uretha, lodges itself there with spines, and quickly becomes engorged with blood to the point it no longer is able to dislodge itself. Removal requires surgery -- the excruciating pain goes without saying.