Many diseases such as cholera, yellow fever, typhus, malaria and various parasitic diseases continue to cause thousands of deaths annually in developing countries despite the existence of therapeutic or prophylactic means of combating them. This situation is aggravated by the fact that the ravages of many diseases are accelerated where hunger and malnutrition exist, and by the fact that adequate health services and medical personnel are often lacking.
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.
In 1990 it was estimated that nearly 10% of the world population suffers from tropical diseases, and the number is expected to increase steadily since remedial action is inhibited by civil unrest in many of the countries where such diseases are most prevalent. Most of the infected live in countries were per capita incomes are less than $400 per year and governments are so poor that they spend no more than $4 per person on their entire health systems.
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).