There are more than 2,000 types of catfish, but there's no need to fear most of them. Except the candiru. It's perfectly fine to fear the candiru. This tiny catfish -- it's only about 0.5 to 3 inches (1.3 to 7.6 centimeters) in length, though they can grow as long as 6 inches (15 centimeters) -- is the stuff of urban legend, except in this particular case, the tale is true [source: Animal Planet]. Candirus are parasitic freshwater catfish that live in the Amazon river, and they just might want to get intimate with your orifices. All of them. Yes, including that one.
These fish like to set up shop inside a host and feed on blood, and they find those hosts because of their acute sense for finding nitrogen. Nitrogen is normally excreted through the gills of a fish, but humans also excrete nitrogen -- in our urine. And since candiru don't know the difference between a gill and a urethra, remember to use the bathroom before you go swimming in the Amazon. It will help reduce your risk of a small catfish anchoring itself inside your urethra.