Influenza A virus （IAV） is a pathogen that causes the flu in birds and some mammals, including humans. It is an RNA virus whose subtypes have been isolated from wild birds. Occasionally, it is transmitted from wild to domestic birds, and this may cause severe disease, outbreaks, or human influenza pandemics.
Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: hemagglutinin （H） and neuraminidase （N）. There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes （H1 through H18 and N1 through N11, respectively）. While more than 130 influenza A subtype combinations have been identified in nature, primarily from wild birds, there are potentially many more influenza A subtype combinations given the propensity for virus “reassortment." Reassortment is a process by which influenza viruses swap gene segments. Reassortment can occur when two influenza viruses infect a host at the same time and swap genetic information. Current subtypes of influenza A viruses that routinely circulate in people include A（H1N1） and A（H3N2）.
Influenza diagnostic tests are based on various technologies from viral isolation to immunodiagnostic and molecular diagnostic methods. Immunodiagnostic tests, especially rapid influenza diagnostic tests （RIDTs） that utilize monoclonal antibodies （MAbs） are often the test of choice due to their ease-of-use and low cost, although the sensitivities of the tests do vary significantly.