Found in up to 60 per cent of all insects, it is harmless to humans. But it has never been found naturally in dengue fever mosquitoes.

The project struggled until the Gates donation arrived.

A PhD student, Conor McMeniman, used super-fine needles to inject 10,000 mosquito embryos with the bacterium. "It was very technical and tedious work," Professor O'Neill said, adding that they had shown the bacterium could halve the adult mosquito's lifespan. Once an insect was infected, the bacterium would spread via its eggs to the next generation.

A pilot release of infected mosquitoes could begin in Vietnam within three years. If no problems are discovered, a full-scale biological attack against the insects could be launched within five years. "We need to make sure the bacteria won't jump into any other species," Professor O'Neill said.

He said Mr Gates was aware of the team's progress. "We are very lucky to have him spending his money the way he does."

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