International cooperation with Third World countries aims at reducing the high morbidity and mortality of the population to a tolerable level. The main health problems are caused by poverty. Thus, the range of diseases in tropical countries can be explained more readily by the socio-economic situation than solely by the climate. Health services, in Africa in particular, have had to reduce drastically their budgets in the last ten years and now have only approximately 1/1000th of the funds usually available in industrialised countries. High population growth reduces the resources available per head, increases infection potential and worsens living conditions. Control strategies must take account of these circumstances in order to achieve the required sustained effect within the framework of primary health care. The example of the control of several infectious diseases, such as schistosomiasis, pneumonia, malaria and AIDS, is used to show that control programmes can be effective but, in the current conditions, can hardly be maintained without outside support. In the future, diseases caused by environmental problems and new life styles as a result of industrialization, urbanization and slum growth will move dramatically into the foreground.