H7N9, a strain of avian influenza not previously known to affect humans, has killed three people and sickened six others in China. Scientists who have examined the virus' genetic material are voicing concerns that the human outbreak could become more serious. "The virus has to a certain extent already adapted to mammalian species and to humans, so from that point of view it's worrisome," said influenza expert Ab Osterhaus. Additionally, the strain appears to cause mostly mild symptoms in birds, making the virus difficult to track and monitor.

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