SARS was an animal disease that jumped the 'species barrier' to infect human beings, according to Science. The magazine described how the SARS virus evolved based on a study conducted by scientists at China's SARS Molecular Epidemiology Consortium and the University of Chicago.

The scientists blame the rapid economic development in China's Guangdong province, where the disease first broke out, for this transition. They found that the rapid development in the area led to 'culinary habits involving exotic animals.' As a result, the virus moved from animals to humans. Six of the 11 outbreaks involved people who had contact with wild animals.

London's The Independent newspaper reported the study of 63 SARS viruses discovered that 'just three major changes to the virus's genetic material' transformed it into a killer disease.

Researchers found that in the first phase of the epidemic the SARS virus was 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.

A final, third phase involved a further set of mutations that allowed the virus to stabilise and get accustomed to spreading still further within its new host species, said Chung-I Wu, professor of ecology and evolution at the University of Chicago.

The second phase occured in January 2003 and resulted in an outbreak of about 130 cases, including 106 patients who acquired the virus in hospital. A doctor at the hospital carried the virus to the Metropole Hotel in Hong Kong where the guests became infected and spread the virus across the world.

The late-phase virus was tentatively traced to one patient who became symptomatic on February 7, 2003.

The study, however, did not determine which animal was the natural 'reservoir' for the virus.

The SARS epidemic killed over 700 people last year.