Human African trypanosomiasis (HAT), or sleeping sickness, is caused by trypanosome parasites that are transmitted by tsetse flies. HAT is found only in sub-Saharan Africa. Two subspecies of Trypanosoma brucei cause disease: T. b. gambiense in West and Central Africa, and T. b. rhodesiense in East Africa.

This life-threatening disease mostly affects poor rural populations, causing significant harm. Travellers to endemic regions may also be at risk of infection.

HAT transmission requires the interaction of humans, tsetse flies and parasite reservoirs (humans, and domestic and wild animals). The animal reservoir is very important in T. b. rhodesiense and less so in T. b. gambiense, although it could explain the long-term endemicity in some foci despite control interventions.