The process of development itself tends to spread disease because the people who arrive in undeveloped areas, where many diseases are carried by animals, become new hosts for the parasites or other disease carriers. Development of water storage and irrigation systems, promotes the dissemination of parasites living in water.
Control of endemic diseases is beyond the reach of many tropical countries, not only because they lack resources but also because of gaps in knowledge and the absence of a proper health technology to make effective use of what is already known. The more important tropical diseases where control is needed include: malaria, schistosomiasis (snail fever), trypanosomiasis (African sleeping sickness and Chaga's disease in South America), leishmaniasis, leprosy and filariasis, including onchocerciasis (river blindness).