Malaria has plagued man since ancient times and nowadays it is estimated to threaten 40% of the world's population, mostly in poor tropical and sub-tropical countries.
No-one knows for sure how many people contract the disease and estimates vary. The latest official study puts the figure at between 350 million and 500 million a year. Malaria is preventable and curable, but can be fatal if not treated promptly.
It kills more than a million people a year - mostly young children in sub-Saharan Africa - and is a factor in many other deaths.
Malaria got its name - literally meaning bad air - because of a once prevalent belief that foul vapours from swamps or marshes were to blame. Proof that microscopic mosquito-borne parasites were the cause came just over 100 years ago. Although malaria has been eradicated from temperate regions, travellers can catch and bring back "imported malaria". And, more rarely, cases of "airport malaria" occur, blamed on mosquitoes aboard aircraft returning from infected areas. Treatments: Despite years of research, no effective vaccine has yet emerged onto the market. But concern over the prevalence of the disease has led to internationally-backed initiatives to tackle malaria - for example Roll Back Malaria, which aims to halve malaria deaths globally by 2010.
People living in areas where malaria is common may get it several times during their lives, and develop some resistance. But young children and pregnant women are particularly susceptible. It is estimated that 2% of children who survive cerebral malaria suffer lasting damage which may impair learning and movement.
Treatment depends on the type of malaria (diagnosed by blood test), where it was contracted and how ill the patient is. A key problem in endemic areas in recent decades has been the growing resistance of the most dangerous parasite species to drugs used to prevent and cure malaria - in particular chloroquine, which is cheap and widely available. New combination treatments containing artemisinin, which comes from a Chinese plant, are recommended by the World Health Organization, but cost far more.
The focus is on improving prevention and enabling effective medication. One key measure is sleeping under bed nets treated with insecticide as malarial mosquitoes generally bite at night. But in general in sub-Saharan Africa such protection has only been available to a tiny proportion of the most vulnerable - young children and pregnant women.
Governments and organisations involved in RBM are now working to get treated nets to those who need them most and claim successes in recent campaigns, notably in Togo.
Listen to "Quest For a Cure", A two part BBC Radio Documentary on Malaria: More informationRoll back malaria fact sheets: What Is Malaria?
Malaria in Africa
Malaria in Pregnancy
Insecticide-Treated Mosquito Nets
Children and Malaria
RBM and Complex Emergencies
Epidemic Prediction and Response
Facts on ACTs (Artemisinin-based Combination Therapies)
Economic Costs of Malaria
Monitoring and Evaluation
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