The disease in cattle, called nagana in Zululand, was linked with trypanosomal parasitaemia and tsetse flies. Nagana occurs in livestock throughout the tsetse belts of Africa. Wild animals are tolerant of trypanosomal infections. Nagana affects individual animals, herds and socio-economic development. In susceptible animals nagana may be acute, but chronic infections are more common. The host-parasite interaction produces extensive pathology and severe anaemia. Clinically affected animals lose condition and become weak and unproductive. Nagana is often fatal and, at herd level, its impact is wide ranging. All aspects of production are depressed: fertility is impaired; milk yields, growth and work output are reduced; and the mortality rate may reduce herd size. Africa has to feed its rapidly growing human population, and animal products are a vital dietary component. However, in most tsetse areas, there is not enough meat and milk. Furthermore, animal draft power is often not available, which limits cultivation and local transport. These factors lower household incomes and retard socio-economic development. Sustainable rural development requires that nagana be controlled. This in turn needs considerable resources, whichever control strategy is adopted.