Almost three weeks after China reported finding a new strain of bird flu in humans, experts are still stumped by how people are becoming infected because many appear to have had no recent contact with live fowl.
The uncertainty adds to challenges the Chinese government faces in trying to control the spread of the H7N9 bird flu virus, which has already killed 17 people and infected 66 others in the country.
Understanding how the virus is spreading is a goal of international and Chinese experts assembled by the World Health Organization as they begin a week-long investigation Friday.
The WHO spokeswoman in China, Helen Yu, says the experts started arriving Thursday and will visit Beijing and Shanghai.
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