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''Influenza A virus'' causes influenza in birds and some mammals, and is the only species of influenza virus A. Influenza virus A is a genus of the ''Orthomyxoviridae'' family of viruses. Strains of all subtypes of influenza A virus have been isolated from wild birds, although disease is uncommon. Some isolates of influenza A virus cause severe disease both in domestic poultry and, rarely, in humans.〔(【引用サイトリンク】title=Avian influenza (" bird flu") — Fact sheet )〕 Occasionally, viruses are transmitted from wild aquatic birds to domestic poultry, and this may cause an outbreak or give rise to human influenza pandemics. Influenza A viruses are negative-sense, single-stranded, segmented RNA viruses. The several subtypes are labeled according to an H number (for the type of hemagglutinin) and an N number (for the type of neuraminidase). There are 18 different known H antigens (H1 to H18) and 11 different known N antigens (N1 to N11). H17 was isolated from fruit bats in 2012. H18N11 was discovered in a Peruvian bat in 2013.〔 Each virus subtype has mutated into a variety of strains with differing pathogenic profiles; some are pathogenic to one species but not others, some are pathogenic to multiple species. A filtered and purified influenza A vaccine for humans has been developed, and many countries have stockpiled it to allow a quick administration to the population in the event of an avian influenza pandemic. Avian influenza is sometimes called avian flu, and colloquially, bird flu. In 2011, researchers reported the discovery of an antibody effective against all types of the influenza A virus. ==Variants and subtypes== Influenza type A viruses are categorized into subtypes based on the type of two proteins on the surface of the viral envelope: :H = hemagglutinin, a protein that causes red blood cells to agglutinate. :N = neuraminidase, an enzyme that cleaves the glycosidic bonds of the monosaccharide, neuraminic acid Different influenza viruses encode for different hemagglutinin and neuraminidase proteins. For example, the H5N1 virus designates an influenza A subtype that has a type 5 hemagglutinin (H) protein and a type 1 neuraminidase (N) protein. There are 18 known types of hemagglutinin and 11 known types of neuraminidase, so, in theory, 198 different combinations of these proteins are possible.〔〔 Some variants are identified and named according to the isolate they resemble, thus are presumed to share lineage (example Fujian flu virus-like); according to their typical host (example human flu virus); according to their subtype (example H3N2); and according to their deadliness (example LP, low pathogenic). So a flu from a virus similar to the isolate A/Fujian/411/2002(H3N2) is called Fujian flu, human flu, and H3N2 flu. Variants are sometimes named according to the species (host) in which the strain is endemic or to which it is adapted. The main variants named using this convention are: * Bird flu * Human flu * Swine influenza * Equine influenza * Canine influenza Variants have also sometimes been named according to their deadliness in poultry, especially chickens: * Low pathogenic avian influenza (LPAI) * Highly pathogenic avian influenza (HPAI), also called deadly flu or death flu Most known strains are extinct strains. For example, the annual flu subtype H3N2 no longer contains the strain that caused the Hong Kong flu. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Influenza A virus」の詳細全文を読む スポンサード リンク
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