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There are three subtypes of influenza A virus, H1N1, H2N2 and H3N2. Prototype virus is named according to type of virus, where the virus was isolated, specimen number and year. For example, A/PR/8/34 indicates that the virus was type A, isolated from specimen number 8 obtained from a patient in Puerto Rico in 1934. This is a specific strain of influenza virus. A/Japan/305/57 was isolated in Japan from specimen number 305 during the Asian flu pandemic in 1957.