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    Fears of Global Bird Flu Outbreak Increase

    Human Pandemic Would Kill Many Millions, But Crucial Facts Still Missing

    Bird Flu Virus Evolving

    Why worry? In the 20th century, there have been three global flu pandemics. The first, the 1918 Spanish flu, was the worst. Coming on the heels of World War I, it killed between 20 million and 40 million people. Milder outbreaks in 1956 and 1968 killed about 1 million each.

    All three times, a bird flu evolved a way to spread among humans. For decades, experts have predicted another outbreak. In 1976, for example, it looked as though swine flu would do the trick. But to the embarrassment of health officials of the time, it fizzled out almost before it began.

    This time, the fears are based on more solid science. Modern genetic techniques have traced major flu outbreaks to birds. Birds can carry 15 known types of type A influenza, the most serious kind of flu virus. Humans -- so far -- get infected with only three of them.

    There are two ways a flu virus can jump from birds to humans. One is evolution; the virus simply mutates into a form that infects humans.

    The evidence so far isn't reassuring. Someone sold chickens that died of bird flu to a Bangkok zoo. Keepers fed the chickens to captive tigers. The virus not only infected the big cats, but also spread easily among them. So far, 147 of the zoo's 441 tigers have died.

    The other way flu viruses figure out how to infect humans is reassortment. When two flu viruses infect the same person or animal, they swap DNA. So far, that hasn't happened. But pigs get infected with human flu viruses. And Chinese health officials report that the bird flu has been infecting pigs for more than a year.

    Early hopes of eradicating the bird flu virus have evaporated. Now the virus is firmly entrenched in domestic and wild birds in Southeast Asia. Could it spread? Recently, officials in Belgium stopped a man getting off a plane from Thailand. They found that he was illegally carrying two hawk eagles. Both birds were infected with H5N1 bird flu. And the veterinarian who examined them came down with a suspicious eye infection, although swab tests were negative for bird flu.

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