At the beginning of the 21st Century, malaria kills more people than Aids. The world's top medics have been unable to stop the humble mosquito from transmitting the deadly disease.

US soldiers are being used as volunteers in the race to find a solution - allowing themselves to be infected by malarial mosquitoes at a military hospital near Washington.

Lieutenant Colonel Gray Heppner of the Walter Reed Army Institute says it is the most damaging infection on earth.

"When you weigh the burden of the disease not only on children but on economic development, it truly has a greater impact than any other infectious disease," he says.

Malaria claims the lives of three children every minute. In Africa, it accounts for a quarter of infant mortality.

Anti-malarial drugs like chloroquine and larium, which were once 95% effective, are now almost useless in parts of the Third World.

Because of global warming, the disease is returning to areas where it had been successfully eradicated.

A nasty bite

Malaria is caused by a tiny parasite that lives in the mosquito's stomach.

Every time the insect bites someone, the parasite enters the person's bloodstream through the mosquito's saliva.

The reason the disease is so widespread is that the parasite is constantly mutating, making it impossible for any known drug to knock it out for long.

The cause of malaria has been known for more than 100 years. The disease has been written off many times only to reappear in a slightly different form.

"The parasite is very clever, in fact so clever that it is almost human," Colonel Heppner says.

He compares the elusiveness of the disease to Professor Moriarity, the arch nemesis of Sherlock Holmes.

"At every turn it evades the immune system. It is inaccessible to anti-bodies, to white blood cells. It constantly changes its cover, its camouflage.

"We have to be more ingenious than it is to pursue it," he says.

Vietnam War lessons

During the Vietnam War, malaria took a huge toll on American soldiers..

The disease reduced the combat strength of some units by half.

That is why the Pentagon spends so much time and money in the hopes of finding a solution.

Army researchers are looking for the Holy Grail in the battle against malaria - a vaccine that would protect their troops and the world against the disease.

The Walter Reed Institute has tried more than 20 different formulae, but each one has failed. The latest trial has been 65% effective

"We are seeing now for the first time the development of malaria vaccines," says Dr Kent Kester who is supervising clinical vaccine trials at the Walter Reed Institute.

"They are showing extreme promise to hopefully protect against malaria, not only here in our small-scale studies, but also in the field."

Private money scarce

Unlocking malaria has turned into an international quest, bringing together researchers from all over the world.

However, private firms are reluctant to risk capital on an expensive hunt to find a vaccine.

At a recent conference which dealt with tropical medicine, pharmaceutical companies were thin on the ground.

Preventing a disease is never as profitable as treating the infected.

The richest man in the world

However, the campaign to find a vaccine is getting a shot in the arm from an unlikely source.

Microsoft chief Bill Gates has donated $6bn to find new vaccines for diseases of the developing world and much of the money will be spent on malaria.

Together with his wife Melinda, he hopes to fill the gap left by the market economy.

"They are very convinced that of all inequities of the human condition today, that which is most inequitable, most unfair and needs attention the most is the problem of diseases of the Third World," said Bill Gates Snr, of the Bill and Melinda Gates Foundation.

Malaria so far has proved to be a much more enduring plague than either smallpox or polio.

With luck and cash, researchers hope to have a malaria vaccine within the first decade of the new millennium.