Scientists have worked out how the virus which caused the world's worst ever flu epidemic infected man.

They believe the virus, which claimed the lives of 50 million people around the world, jumped from birds to humans.

The breakthrough, published in Science, should help doctors identify which future bird viruses pose a threat to man at an earlier stage.

But the National Institute for Medical Research team warns viruses cannot be stopped from crossing between species.

They also say their work is unlikely to aid the current fight against avian flu in the Far East as knowing the structure of a virus is not enough to block its progress.

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