Human influenza A viruses usually refers to those influenza A subtypes that have spread widely among humans. In the year 2006 H5N1 influenza A subtypes was circulating among humans.
There are many different subtypes of type A influenza viruses. These subtypes differ based on differences in two main proteins on the surface of the influenza A virus (hemagglutinin [HA] and neuraminidase [NA] proteins).
There are 16 known HA subtypes and 9 known NA subtypes of influenza A viruses. Many different combinations of HA and NA proteins are possible. Each combination represents a different subtype. All known subtypes of influenza A viruses can be found in birds.