While mosquito-borne diseases like malaria emerge from stagnant ponds and puddles, it is fast-running water, in conjunction with the blood-sucking black fly, that creates a breeding ground for river blindness.
Black flies carry a microscopic nematode larva that, thanks to a penny-sized nodule under the skin, can be easily detected after it has grown and reproduced inside a human host, but not so easily defeated.
This program visits an isolated village in Cameroon where river blindness has struck again and again.
Despite the tragedy, the community is becoming a model for a program that distributes and tracks compliance with Mectizan, a drug that kills the young worms but must be taken for years.
This film is part of the KILL OR CURE series of 57 x 25-minute documentaries produced for the BBC. These films can be bought individually or as a series.