The World Health Organization (WHO) said it was now more than three weeks since the last case of Severe Acute Respiratory Syndrome was placed in isolation, among a cluster of nine cases linked to the National Institute of Virology in Beijing as the likely source. It urged all Member States to review biosafety practices of institutions and laboratories working with the SARS coronavirus.In March a 26-year-old laboratory researcher came down with the newly emergent disease, which last year killed 774 people and infected more than 8,000 worldwide, the vast majority of them in China. The researcher’s mother, who cared for her, died and a nurse fell ill. The other cases were linked to these patients or the institute, where a second researcher became ill.WHO experts and the Chinese authorities are still trying to determine the exact cause of the outbreak, and preliminary findings have yet to identify a single infectious source or single procedural error at the Institute. It is conceivable that an exact answer may never be determined, WHO said.Neither researcher is known to have directly conducted experiments using live SARS virus, but WHO said investigators have serious concerns about biosafety procedures at the Institute, including how and where procedures using the were carried out and how and where the samples were stored.During and after the SARS outbreak of 2003, a large number of specimens were collected from possible human cases, animals and the environment. These specimens, which may contain live virus, are still kept in various laboratories around the world.