Live poultry markets provide an ideal environment for viral reassort-ment and interspecies transmission of avian influenza viruses. Ducks
and quail can act as mixing vessels to facilitate reassortment of influ-enza viruses of different subtypes, which may result in the generation
of novel reassortant avian influenza viruses and the selection of mutants that may expand their host range to mammals.nH7N9
has a five-amino-acid deletion in the NA stalk that may be associated with increased virulence.Influenza viruses with a long NA stalk have
a selective advantage in aquatic birds.