“Genetic tuning” of avian influenza virus host adaptation from birds to humans

Emerging viral infections are a growing threat to human health and often have originated from zoonotic diseases[1].Viruses infecting animals can cross the interspecies barrier and infect humans by the evolution of genetic variants with more fit replication in mammals,which has been defined as“genetic tuning”[2].All the influenza A virus subtypes have been isolated fromwild birds.Influenza A viruses cross the avian-human species barrier by acquiring genetic mutations or reassortments that enable them to adapt to humans[3].Several avian influenza viruses(AIVs)(e.g.,H5N1,H7N7,H9N2,H7N9,H5N6 and H7N4)can infect humans and cause respiratory diseases with a range of severity[4].