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Scientists have identified thestages in the evolution of the Sars virus that turned it from a simple animal infection into a lethal human disease.

A study of the genes of more than 60 Sars viruses has found that it took just three major changes to the virus's genetic material to transform it into a highly effective killer of people.

During the earliest phase of the epidemic the Sars virus infected about 3 per cent of those who came into direct contact with it, according to the study. Within a few months the proportion was 70 per cent.

Chinese scientists, working with a team at the University of Chicago, analysed the Sars virus from the early, middle and later phases of the 2002-03 epidemic. The researchers found that the first phase of the epidemic involved a virus virtually identical to that found in wild animals but it mutated to cause a second phase, during which the virus spread from human to human.