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Influenza viruses are a group of RNA viruses from Orthomyxoviridae family. These are 3 in number namely: Influenzavirus A, Influenzavirus B and Influenzavirus C. Each of these genus has only one species namely influenza A virus, influenza B virus and influenza C virus. Among the three, type A causes severe disease and is the most virulent as human pathogen.
Influenzavirus B, in contrast to influenzavirus A is known to infect only humans and seals. The mutation in type B is 2-3 times slower compared to that of type A, however its fast enough to have a lasting immunity against it in humans. Influenza C virus infects pigs and humans. It is rare in comparison to A and B, although it could also be severe enough to cause endemic.
Coming to Influenza virus A, these can be categorized in different HN types. H and N, here stand for the two proteins called Hemagglutinin and Neuraminidase. These are present on the surface of the virus and help in its adhesion and penetration to the host cell. Sixteen types of Hemagglutinins and 9 types of Neuraminidases are known, giving rise to 144 combinations of the subtypes of influenza A virus. Common examples are H1N1, H3N2 (Swine flu) and H5N1 (bird/avian flu). All these subtypes infect and spread widely among humans resulting into pandemics.