No. 126: Nov-Dec 1999
Of course, most freshwater fish do swim upstream in the usual way, but not the candiru. It is partial to very specific streams.
This small, slim, scaleless species of catfish inhabits the Amazon where it preys on other fish, often by invading their gills and feasting on blood and tissue. They also aim much higher on evolution's ladder: they are the only known vertebrate parasites of humans.
It is when they prey upon humans that they swim up the wrong streams -- at least their victims think so! Their technique is simple. They detect human urine released by swimmers and follow it right to the source. They don't stop there but insert themselves right into the penis and keep going, sometimes all the way to the bladder. Once inside the penis they erect their spines and cannot be extracted except by surgery. Left alone, they are not only excruciating to the unwise bather but can eventually be fatal. Surgeons have successfully extracted them from the bladder, but in remote areas penis amputation is the only answer!
(Warren, Nicholas; "In Mare Internum," Fortean Times, p. 14, September 1999. Title translation: "Within the Inner Sea.")