听力原文： The WHO has confirmed that at least three people have died of bird flu in Vietnam, the same virus that first emerged in Hong Kong in 1997 and killed six people. Hundreds of thousands of chickens have been infected by the disease in Vietnam, South Korea and Japan. The WHO says the virus, H5-N1, is the same one that suddenly jumped from chicken to humans in 1997, killing six of the 13 people it infected in Hong Kong. Thousands of chickens have been infected in Vietnam, and the government has asked the U.N. agency to help contain the disease. In South Korea, over a million chickens have either died from the flu or been culled. Japan on Monday reported its first outbreak of H5-N1, with about 6,000 chickens infected. When the link between sick chickens and the HS-N1 virus in humans was discovered in Hong Kong in 1997, the government ordered all chickens in the city killed. They also suspended imports of birds from Mainland, China. How many people have been infected by bird flu in Hong Kong in 1997?