It has been more than a month since the last case was placed in isolation in China's latest SARS outbreak, prompting the World Health Organization to declare that the chain of human-to-human transmission appears to have been broken.
However, WHO experts and the Chinese authorities are still trying to determine the exact cause of the outbreak. The investigation has centred primarily on the National Institute of Virology in Beijing where experiments using live and inactivated SARS coronavirus have been carried out. Two researchers at the Institute developed SARS in late March and mid-April. The outbreak was reported on April 22 and the institute was closed a day later.