Global health experts were last night trying to identify a mystery flu virus which has struck down 50 staff at Prince of Wales Hospital in Sha Tin, claiming 23 victims yesterday alone. Of the 23, eight were diagnosed with atypical pneumonia, bringing to 10 the number afflicted with pneumonia since the outbreak began on March 8. Health authorities said it was so far confined to Prince of Wales and it appeared the virus had spread from one ward. All other public hospitals have been put on alert. Deputy Director of Health Leung Pak-yin said: 'So far, what we have assessed is that these are pneumonia cases acquired in hospital and have not spread to the community.' The World Health Organisation in Geneva is studying the outbreak. Its spokesman Dick Thompson said tests carried out so far had been unable to identify the outbreak or link it to outbreaks in Guangdong and Hanoi, Vietnam. Three hundred people fell ill in Guangdong during an outbreak of pneumonia last month.