As the number of infections passed 70 and the death toll reached 14, the H7N9 avian influenza strain raised new worries in China as public health experts announced the case of a 4-year-old asymptomatic carrier of the virus. The development poses concerns about monitoring the virus, which was not known to infect humans until the recent outbreak began. The pathogen is already difficult to track because it does not lead to serious illness in avian species.

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