Influenza A[edit]
Main article: Influenza A virus
Diagram of influenza nomenclature
Influenza A viruses are further classified, based on the viral surface proteins hemagglutinin (HA or H) and neuraminidase (NA or N). 18 HA subtypes (or serotypes) and 11 NA subtypes of influenza A virus have been isolated in nature. Among these, the HA subtype 1-16 and NA subtype 1-9 are found in wild waterfowl and shorebirds and the HA subtypes 17-18 and NA subtypes 10-11 have only been isolated from bats.[29][30]
Further variation exists; thus, specific influenza strain isolates are identified by the Influenza virus nomenclature,[31] specifying virus type, host species (if not human), geographical location where first isolated, laboratory reference, year of isolation, and HA and NA subtype.[32][33]
Examples of the nomenclature are:
A/Brisbane/59/2007 (H1N1) - isolated from a human
A/swine/South Dakota/152B/2009 (H1N2) - isolated from a pig
The type A influenza viruses are the most virulent human pathogens among the three influenza types and cause the most severe disease. It is thought that all influenza A viruses causing outbreaks or pandemics originate from wild aquatic birds.[34] All influenza A virus pandemics since the 1900s were caused by Avian influenza, through Reassortment with other influenza strains, either those that affect humans (seasonal flu) or those affecting other animals (see 2009 swine flu pandemic).[35] The serotypes that have been confirmed in humans, ordered by the number of confirmed human deaths, are: