WHAT IS IT: A life-threatening parasitic disease transmitted by mosquitoes that jousts with AIDS as the world's biggest health problem. Its name -- "mal aria" ('bad air' in Italian) -- comes from the myth that it comes from breathing in air from stagnant marshes. In 1880, scientists isolated a one-cell parasite called a plasmodium that is the real cause of malaria. The disease occurs in poor tropical countries that are home to 40 percent of the world's population. It was eliminated from temperate climates by the middle of the 20th century, although some experts fear it will stage a comeback through global warming.

THE TOLL: Malaria kills over a million people a year, or about 3,000 a day. Ninety percent of cases are in sub-Saharan Africa. At least 300 million people suffer from acute malaria each year, often suffering lasting effects. Children aged under five, pregnant women and their unborn children and the elderly are the most vulnerable.

SYMPTOMS: Fever, headache, vomiting and other flu-like symptoms appear about nine to 14 days after the parasite is passed on, the duration varying according to the type of plasmodium. Drugs are then needed to treat the symptoms and kill the parasites. Malaria can kill by destroying red blood cells, causing anaemia, or clog the blood capillaries that provide oxygen to the brain, a condition called cerebral malaria. The most lethal of the four parasite strains is Plasmodium falciparum.

HOW IT SPREADS: Via a female mosquito of the Anopheles gambiae strain, which feeds almost exclusively on human blood and hands on the parasite in its saliva when it takes a blood meal. Once inside the body, the parasite undergoes several complex changes as part of its life cycle. It evades the immune system, infects the liver and red blood cells and finally develops into a form that is able to infect a mosquito again when it bites an infected person.

TREATMENT: As yet, there is no cure or vaccine for malaria, although a new class of drug, artemisinin-class, is highly effective. Swift use of drugs to treat symptoms and kill the parasites in an infected person can significantly reduce the death toll. The cheapest drug, chloroquine, is rapidly losing its effectiveness because the P. falciparum parasite is becoming resistant to it.

PREVENTION: Draining stagnant pools and water butts, where mosquitoes breed, and using insecticide-treated mosquito bed netting can stop more than half malaria transmissions in high-risk areas.

ECONOMIC COST: Malaria costs Africa more than 12 billion dollars a year. The continent's gross domestic product would be a third greater today if malaria had been eliminated in the 1960s, using DDT. That insecticide was withdrawn because it is a dangerously accumulative pollutant.

TACKLING MALARIA: In 1998, the Roll Back Malaria (RBM) Global Partnership was launched by the World Health Organization, UNICEF, UNDP and the World Bank, with the goal of halving the number of deaths from malaria by 2010. Two billion dollars are needed annually to achieve this goal, according to the RBM's estimates. Only about 200 million is likely to be available this year.

Sources: World Health Organisation (WHO); Science; Nature; Harvard University study; WHO's Roll Back Malaria Department.

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