Hong Kong ordered more than half a million primary and kindergarten students to stay home from school for two weeks starting Thursday after three schoolchildren died amid recent outbreaks of the flu. The government asked one of the territory's top scientists to investigate the three deaths. But the World Health Organization said only two of the children tested positive for the flu, and both had other diseases as well. The outbreak has not been linked to bird flu, which has been detected in birds in Hong Kong. Bird flu remains difficult for humans to catch, but scientists fear the virus that causes it could eventually mutate into a form that spreads easily among humans. The government has ordered all kindergartens, primary and special education schools closed for two weeks starting Thursday, Health Secretary York Chow said.