Emergence and development of H7N9 influenza viruses in China.

•H7N9 virus was generated through sequential reassortments in ducks and chickens.•This virus originated from eastern China and has spread to over 20 provinces.•Regionally distinct lineages have been established with different genotypes.•Viruses present at live poultry markets fueled the recurrence of human infections.•Chickens served as the source of human infections in each outbreak wave.