Ducks and geese are natural carriers or reservoir hosts of the influenza virus. A carrier usually means the animal does not appear ill, and remains unharmed despite the virus replicating in the animal. This natural resistance is genetic based and recent studies point to at least one candidate duck gene that helps prevent disease in the duck.
The path to influenza virus infection in humans involves a number of steps including transmission of the virus from ducks to chickens, mutation of the virus, reassortment, and adaptation in humans. Chickens are infected, become ill and often die. They act as amplifying hosts and spread the virus to humans. Breaking this chain of events by making chickens more resistant to the virus has been recently demonstrated.