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During the spring of 2013,a novel avian-origin influenza A (H7N9) virus emerged and spread in Shanghai.On May 9,2013,33 lab-confirmed cases (2 children and 31 adults) have been reported,among whom 13 died.Among the 31 adult cases of H7N9 avian influenza A virus infections in humans,80.6％ (25/31) were older men (average age 65 years).Clear history of poultry exposure was found in eight patients (8/33,24.3％) and the remaining 21 cases had been suspected poultry exposure.Clustered cases were found in two families (two cases from a father and his son,and the other two cases from a wife and her husband).However,no evidence of human-to-human transmission had been identified.Avian influenza is one of the most dangerous contagions in poultry worldwide,and avian influenza A virus is the major pathogen responsible.Transmission from poultry to humans has now been proven.Some gene segments of the strain have mutated,and further mutations might result in human-to-human transmission.