The SARS virus could be the result of a merger between viruses carried by birds and mammals, say researchers. The finding is based on a genetic analysis of the coronavirus, which causes the disease, and others that are closely related.
A similar phenomenon is responsible for the emergence of new types of flu virus.

Sars killed 774 people and infected as many as 8,000 in a world-wide outbreak earlier this year. China and Hong Kong bore the brunt of its effect, but Taiwan, Singapore and Canada also recorded many deaths.

The virus was eventually identified after a concerted international effort as a new kind of coronavirus. Such viruses usually cause a range of veterinary diseases - but usually nothing worse than the common cold in people.

About half the DNA in the SARS virus looked like coronavirus sequences taken from mammals. But the other half looked liked coronoviruses normally found in birds. And a key gene in the virus, known as the spike gene, seemed to be a mix of the two. The spike gene is thought to control the virus' ability to infect cells.

SARS has not been seen since it was brought under control in June but health officials are watching warily for it to re-emerge.

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