Hong Kong ? Hong Kong on Wednesday ordered more than half a million primary and kindergarten students to stay home for two weeks because of a flu outbreak in one of the world’s most densely populated cities.

The government also asked one of the territory’s top scientists to investigate the deaths of three children. But the World Health Organization said only two of the children tested positive for the flu, and both had other diseases as well. The WHO added Thursday that there was no sign that the situation in Hong Kong was different from a regular seasonal outbreak.

The outbreak has not been linked to bird flu, which was 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 beginning today, Health Secretary York Chow said.