The candirú is one of the scariest of all vampire animals, but you're probably safe as long as you vow never to urinate in the Amazon or Orinoco rivers. Those are the only known habitats for this tiny, parasitic catfish, which attacks other fish by swimming into their gills — and, on rare occasions, attacks a person by swimming into his or her urethra.
In one documented case, a candirú swam out of the Amazon River, up a local man's urine stream and into his urethra (illustrated here, not graphically), where it tried to feed on blood and tissue before it died. This possibility had been rumored for centuries, but Brazilian surgeons finally verified it in 1997, finding a dead, 6-inch candirú lodged in the victim's urethra. They removed it, and he reportedly made a full recovery.
To watch a candirú hunt, and to see how it can wind up inside the wrong host, check out the video below: