The prevalence of human African trypa nos omiasis (HAT) rose steadily after the 1960s, but is now on the wane. WHO estimates that 300 000 to 500 000 people are affected, but there are no accurate figures.
The nature of this disease is a challenge. HAT occurs in two forms (chronic and acute), in two different stages (early and late), and is fatal unless treated. Epidemics develop if there is no intervention. Trypanosoma brucei gambiense, found in central and West Africa, causes chronic infection; a person can be infected for months or years without obvious symptoms, which emerge only when the disease has reached the late stage.