Malaria is spread by mosquitoes
The number of cases of the deadliest form of malaria across the world could be twice as high as previously predicted, researchers suggest.
An Oxford University team, writing in Nature, estimated there were over half a billion cases of Plasmodium falciparum malaria globally in 2002.
The figure is up to 50% higher than estimates from the World Health Organization.
Two thirds of cases occurred in Africa, predominantly affecting under-fives.
The new figures are 200% higher for areas outside Africa.
The study suggests that, in total, 2.2 billion people are at risk from malaria.
The researchers say this could be because the WHO's reliance on all centres in a particular country reporting all cases of the disease in order to collate incidence data was less certain than the method they used.
The WHO had set a target to halve deaths by 2010, but resistance to drugs is threatening that plan.
'Need to spend wisely'
Scientists from the University of Oxford, based at the Kenyan Medical Research Institute/ Wellcome Trust Laboratories used contemporary and historical epidemiological, geographical and demographic information to model where people live, the likelihood of infection from malaria parasites and susceptibility to developing the disease.
They also used Geographic Information Systems and data from earth orbiting satellites.
Professor Bob Snow, who led the research, said: "We have taken a conservative approach to estimating how many attacks occur globally each year but even so the problem is far bigger than we previously thought.
"We have taken a science-driven approach to working out who is at risk, where they are located and what their chances would be of developing an attack of malaria.
"Our work has demonstrated that nearly 25% of worldwide cases occur in South East Asia and the Western Pacific - whereas most people regard Plasmodium falciparum disease a problem particular to Africa."
He added: "Getting numbers right is important. Not knowing the size of the problem limits our ability to articulate how much money we need to tackle the problem - not knowing where the problem is located means you can't spend wisely.
"This is particularly important for new drugs needed to fight malaria.
"These are expensive and difficult to produce and production capacity and financing can be driven by speculation, poor data or simply best-guesses."
Professor Nick White, Director of the Wellcome Trust's South East Asia unit, said : "If we are going to Roll Back Malaria then we need to know the size of the problem- and where it is.
"Falciparum malaria has increasingly been thought of as an African problem.
"These estimates challenge that notion and suggest there is a lot more falciparum malaria in SE Asia than previously thought."
And Dr Richard Feachem, Executive Director of the Global Fund to Fight AIDS Tuberculosis and Malaria, said : "This study is important.
"Many have believed that existing data grossly under-estimates malaria, morbidity and mortality in Africa and Asia. We now have confirmation of this.
"Accurate numbers are essential to the Global Fund, which is charged by the international community with financing the counter-attack against malaria, especially through purchases of effective drugs and insecticide-treated bednets.
"We must now significantly increase our estimates of resources needed."