H7N9 - What Is It?

There are 3 types of influenza viruses: A, B and C.

Type A viruses can infect humans, birds, pigs, horses and other animals, and wild birds are the natural hosts for these viruses. They are subject to gradual mutations (known as antigenic drift) as well as sudden changes in their surface proteins (known as antigenic shift). They are very dynamic and are continuously changing. Current vaccines may not be able to work to prevent or cure the disease, hence global surveillance is critical to monitor for their evolution. Because of their variability, type A viruses can cause major pandemics (spread over large areas).
Type B viruses can usually only infect humans. They undergo only gradual antigenic drift and cause only centralised epidemics (spread over small localised areas).
Type C viruses can only infect and cause mild illness in humans and are antigenically stable (i.e. they do not undergo any antigenic drift and antigenic shift). They cause only sporadic diseases and do not cause epidemics or pandemics.

Influenza type A viruses are further classified into subtypes on the basis of two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). There are 16 different hemagglutinin subtypes (H1 to H16) and 9 neuraminidase subtypes (N1 to N9). Various combinations of hemagglutinin (H) and neuraminidase (N) subtypes are possible. Following are some known influenza type A viruses:

H1N1 - Spanish flu pandemic of 1918 and swine flu pandemic of 2009
H2N2 - Asian flu pandemic of 1957
H3N2 - Hong Kong flu pandemic of 1968
H3N8 - frequently found in horses
H5N1 - bird flu
H5N2 - highly capable to produce diseases in chickens
H7N7 - 2003 poultry epidemic
H7N9 - new bird flu in China of 2013

Influenza type B and type C viruses are not classified into subtypes.

The influenza virus recently found in China is rightly named as Type A H7N9 virus because it is of the Type A influenza virus with hemagglutinin 7 and neuraminidase 9 subtype proteins. It was first reported to have infected humans in March, 2013.

(Information are mostly sourced from 'Influenza Viruses, November 2005, Centre of Disease Control USA')