Of all disease-transmitting insects, the mosquito is the greatest menace, spreading malaria, dengue and yellow fever, which together are responsible for several million deaths and hundreds of millions of cases every year. Mosquitos also transmit lymphatic filariasis and Japanese encephalitis. Other insect species carry a variety of diseases. Sleeping sickness is spread by the tsetse fly, with 55 million people at risk. The leishmaniasis group of diseases is spread by sandflies, with 350 million people at risk. Another 100 million in Latin America are at risk of Chagas disease, spread by household bugs. Onchocerciasis, or river blindness, is carried by blackflies, and plague by fleas.
Malaria is endemic in 91 countries, with about 40% of the world's population at risk. By undermining the health and working capacity of hundreds of millions, it is closely linked to poverty and stunts social and economic development. Up to 500 million cases occur every year, 90% of them in Africa, and there are up to 2.7 million deaths annually.
Dengue is the world's most important mosquito-borne virus disease, with 2500 million people worldwide at risk of infection and 20 million cases a year in more than 100 countries. In 1995, the worst dengue epidemic in Latin America and the Caribbean for 15 years struck at least 14 countries, causing more than 200 000 cases of dengue fever and almost 6000 cases of the more serious dengue haemorrhagic fever.
Many major cities of the world, especially in the Americas, are at risk of potentially devastating epidemics of yellow fever because they are infested with Aedes aegypti mosquitos which can transmit the disease. Lymphatic filariasis (elephantiasis) infects about 120 million people in tropical areas of Africa, India, South-East Asia, the Pacific Islands and South and Central America.
Among diseases spread by other insects, leishmaniasis occurs in 88 countries, and its spread is accelerated by road building, dam construction, mining and other development programmes that bring more people into contact with the sandflies that transmit the causative parasite. Sleeping sickness affects 36 countries of sub-Saharan Africa. Onchocerciasis affects some 17.6 million people in Africa, and a smaller number in Central and South America. At least 16 million people in Latin America are infected with Chagas disease. Plague continues to strike relatively small numbers of people in Africa, the Americas and Asia.