H7N9, the newly identified avian influenza strain that has so far infected scores of people and killed more than 20, passes between birds and humans more easily than the H5N1 strain that surfaced in 2003, according to the World Health Organization. After a weeklong investigation, a team of global researchers believe exposure to live ducks and chickens at poultry markets is associated with transmission. Of concern is the fact that birds generally don't display clear symptoms, but closing some poultry markets appears to have decreased new human cases.

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