influenza a virus subtype h9n2 background -

virus h9n2 h5n1 poultry h5n8 influenza h7n9 sick symptoms avian flu outbreaks viruses bird pandemic pathogenic h7n3 h7n7 zoonotic vaccine disease health mortality epidemiology infection mutations epidemic genetic hemagglutinin neuraminidase subtype medicine chickens strain genetics diseases h1n2 antibody biochemistry coronavirus covid19 hygiene medical sample microbiology molecular tests vector background file