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Photo by: C.S. Goldsmith, T. Rowe

An electron microscope image provided by the Centers for Disease Control and Prevention shows the H7N9 virus which can take on a variety of shapes, April 15, 2013. 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 when many appear to have had no recent contact with live fowl and the virus isn't supposed to pass from person to person. (Associated Press)

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