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What is avian influenza? Avian influenza is an infectious disease of birds caused by type A strains of the influenza virus. The disease, which was first identified in Italy more than 100 years ago, occurs worldwide. The risk from avian influenza is generally low to most people, because the viruses do not usually infect humans. However, confirmed cases of human infection from several subtypes of avian influenza infection have been reported since 1997. Influenza type A viruses are divided into subtypes and are named on the basis of two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA). For example, an “H7N2 virus” designates an influenza A subtype that has an HA 7 protein and an NA 2 protein. Similarly, an “H5N1” virus has an HA 5 protein and an NA 1 protein. There are 16 known HA subtypes and nine known NA subtypes. The avian influenza H5N1 virus Influenza A (H5N1) virus, also called “H5N1 virus,” is an influenza A virus subtype that occurs mainly in birds, is highly contagious among birds, and can be deadly to them. This is the virus that has infected and/or killed dozens of humans in Asia since 1997. Unlike seasonal influenza, in which infection usually causes only mild respiratory symptoms in most people, H5N1 infection may follow an unusually aggressive clinical course, with rapid deterioration and high fatality. Primary viral pneumonia and multi-organ failure have been common among people who have become ill with H5N1 influenza.