Influenza A virus (English: Influenza A virus, often referred to as influenza A) is a kind of influenza virus, including multiple subtypes, which can infect wild birds, domesticated poultry, pigs, horses and humans and other mammals and cause influenza. The virus is the only species in the family Orthomyxoviridae, the genus Influenza A. Strains of all subspecies of influenza A virus have been extracted from wild birds, but the birds themselves are rarely affected. Some isolates of influenza A virus cause disease in poultry and, to a lesser extent, humans. [1] The virus can sometimes infect poultry through wild waterfowl, and this can lead to human influenza outbreaks. [2][3]

People have developed a vaccine against influenza A virus, and many countries have stockpiled a lot of vaccines in order to be able to respond quickly when bird flu breaks out. In 2011, researchers reported finding antibodies effective against all subtypes of influenza A. [4]

The famous influenza pandemics in history were all caused by influenza A virus, including Spanish flu in 1918, Asian flu in 1957, Hong Kong flu in 1968, Russian flu in 1977, H5N1 bird flu in 2003, H1N1 flu in 2009, 2013 flu H7N9 influenza, etc.