Ducks were the melting pot of viruses that led to the new bird flu emerging in China early this year, according to Chinese scientists tracking the evolution of the virus.
Ducks picked up viruses from migrating birds and passed them onto chickens.
The study, published in the journal Nature, showed humans were probably then infected with H7N9 due to contact with chickens at live poultry markets.
There have been 133 human cases of the bird flu and 43 deaths.
The team, including researchers at the Shantou University Medical College, were trying to trace the root of the outbreak.