The Trypanosoma parasite exists in two sub-species in Africa. The World Health Organisation (WHO) estimates that the disease threatens 70 million people in the poorest underdeveloped countries in the world.
T. brucei gambiense is widespread in the western and central parts of Africa. Its host fly prefers shady river banks. The T. brucei gambiense parasite can apparently only affect humans, and so there may be a chance of bringing the disease under control.
T. brucei rhodiense exists in eastern and southern Africa, particularly in areas with large numbers of cattle. Unfortunately, this parasite can also infect a number of mammals, so that there's a 'reservoir' in the animal population that's difficult to eradicate.
Epidemic areas are noted above. In several provinces in these countries, it's estimated that up to 20 per cent of the population are infected by the disease. It has a serious social and economic impact by affecting the workforce and resources and is a major obstacle to development in these areas.
Considerable attempts have been made to control the parasite since the 1930s, and the number of infected people had been falling.
But in the last 30 years, increasing poverty and a decreasing number of surveillance programmes have had undesirable consequences, and epidemics have been seen both in known endemic areas and in areas where it had been thought that the fly and parasite had been eradicated.