Ann Reid explains how she "caught" the flu

It is the first step towards finding out why that virus strain was so deadly. It is also a step towards creating better protection against future outbreaks, which the scientists warn are inevitable.

The genetic information was painstakingly pieced together from virus fragments taken from the body of an Inuit woman released from the permafrost of Alaska's tundra in 1997. The preserved lungs of two US soldiers were also used.

Ann Reid and colleagues at the Armed Forces Institute of Pathology, Washington DC, USA, carried out the study. She told BBC News Online the fact that decomposition had broken up the virus had advantages and disadvantages.

"If we had it whole, we could sequence it in a week. In pieces it will take us four years, but this way does mean there is no risk at all.

Lung tissue from soldiers who died in 1918

"We hope there will be some clues in the genetic structure that we could then look for in new emerging viruses. You could then possibly design drugs or vaccines in advance to target the particular changes that makes it so lethal."

The team's work shows that the 1918 virus, which killed by filling the lungs with fluid and causing haemorrhaging, had adapted to pigs and humans several years before the outbreak.

This is unusual as most pandemics, including those in 1957 and 1968, transfer directly from birds. However, the 1918 virus was the most closely related to "bird flu" of all those found in humans.

The work is a triumph for Dr Johan Hultin, whose intrepid expedition to Alaska recovered the tissue from the Inuit woman's corpse. He was successful because the woman had been very fat and was therefore slow to decay. Dr Hultin's adventures were the subject of a recent BBC Horizon documentary.

The gene the team sequenced is crucial in the infection process. It produces a protein called hemagglutinin which sits on the surface of the virus particle and grabs cells to infect them. Because it is on the surface, hemagglutinin is also the primary target of the body's immune system.

The research is published in the Proceedings of the National Academy of Sciences.