Human and animal trypanosomosis, transmitted by tsetse flies constitutes one of the major health and agricultural livelihood constraints of sub-Saharan Africa. The trypanosomoses are diseases of humans and domestic animals that result from infection with parasitic protozoa of the genus Trypanosoma. Trypanosomes belong to the Order Kinetoplastidae in the family Trypanosomatidae. The further classification is based on the mode of transmission via faeces or saliva. Members of the former group are included in the section Stercoraria, and the latter in the section Salivaria.

Cyclical transmission of trypanosomes is accomplished through the bite of an infected tsetse fly. Tsetse flies get the infection when feeding on an infected animal. After completion of the parasitic cycle in the fly (15–21 days), the tsetse fly becomes infective and may remain infective for the rest of its life. Transmission occurs in the early stage of the blood feeding, when the fly injects some saliva before sucking the blood of its host. Mechanical transmission by means of biting flies, especially tabanids and stomoxys, but possibly other biting insects (including tsetse flies) are limited to T. vivax. Mechanical transmission can occur when interrupted feeding is re-started on a new host. This is efficient among a group of animals but has little chance to occur over a distance.  The exception to the mentioned modes of transmission is venereal transmission of Trypanosoma equiperdum in horses.