This genus has one species, influenza A virus. Wild aquatic birds are the natural hosts for a large variety of influenza A. Occasionally, viruses are transmitted to other species and may then cause devastating outbreaks in domestic poultry or give rise to human influenza pandemics. The type A viruses are the most virulent human pathogens among the three influenza types and cause the most severe disease. The influenza A virus can be subdivided into different serotypes based on the antibody response to these viruses. The serotypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:

	H1N1,	which caused Spanish Flu in 1918, and Swine Flu in 2009
	H1N2,	endemic in humans, pigs and birds
	H2N2,	which caused Asian Flu in 1957
	H3N2,	which caused Hong Kong Flu in 1968
	H5N1,	which caused Bird Flu in 2004
	H7N2
	H7N3
	H7N7,	which has unusual zoonotic potential
	H10N7