In humans, tsetse transmitted trypanosomiasis is called sleeping sickness. Human African trypanosomiasis is vector-borne, and spread through the bite of the tsetse fly. Other forms of human trypanosomiasis also exist but are not transmitted by tsetse. Infection occurs when a vector tsetse fly bites a mammalian host. Larger flies such as tsetse flies and screwworms cause significant economic harm to cattle. Wild mammal populations increased rapidly, accompanied by the tsetse fly. Twenty-three extant species of tsetse flies are known from Africa. Tsetse pupae are under 1 cm long. Most tsetse flies are physically very tough. The areas occupied by the tsetse fly were largely barred to animal husbandry. Most of the 37 countries infested with tsetse are poor, debt-ridden and underdeveloped. The internal anatomy of tsetse is fairly typical of the insects. "T. brucei" is found where the tsetse fly vectors are prevalent. The biology of tsetse is relatively well understood by entomologists. Trypanosomiasis could, in future be prevented by genetically altering the tsetse fly.