Malaria, in second place, sees fewer victims, but greater difficulty of control.

• Position: Includes related article on death statistics

Malaria is the world's most deadly tropical parasitic disease which kills more people than any other communicable disease, except tuberculosis. Although the geographical area affected by malaria has shrunk considerably over the past 50 years, control is becoming more difficult, and gains have been eroded. The disease is transmitted by Anopheline mosquitoes, the number and type of which determine the extent of transmission in a given area. Increased risk of the disease is linked with changes in land use related to activities like road building, mining, logging and agricultural and irrigation projects, particularly in frontier areas like the Amazon, and in South-East Asia. Other causes of its spread include global climatic change, disintegration of health services, armed conflicts and mass movements of refugees. The emergence of multi-drug resistant swains of parasite is also exacerbating the situation. As a result of the explosion of international travel, imported cases of malaria are now more frequently registered in developed countries. Malaria is re-emerging in areas where it was previously under control or eradicated, e.g. in the Central Asian Republics of Tajikistan and Azerbaijan, and in Korea.

Malaria endemic countries are some of the poorest in the world and results in costs to the affected countries in terms of the control of the disease and lost workdays - estimated to be I to 5 per cent of the gross domestic product in Africa. For the individual, costs include the price of treatment and prevention, and lost income. In rural areas, the rainy season is often a time of intense agricultural activity when poor families earn most of their annual income. Malaria can make these families poorer. The disease can lead to chronic absenteeism and impairment of learning ability in affected children. Urban malaria is increasing due to unplanned development around large cities, particularly in Africa and South Asia.

According to WHO, "global warming" and other climatic events such as "El Nino" also play a role in increasing the risk of the disease. Malaria has now spread to highland are of Africa, while El Nino events have an impact on malaria because the associated weather disturbances influence vector-breeding sites, and hence the transmission of the disease. Quantitative leaps in malaria incidence, coincident with ENSO (El Nino/Southern Oscillation) events, have been recorded around the world - in Bolivia, Colombia, Ecuador, Peru, Venezuela...