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Influenza (flu) is a universally common epidemic illness caused by several subtypes of type A or by type B influenza virus. (Type C influenza virus exists and produces a mild respiratory illness but is not believed to cause epidemics.) The most common subtypes of influenza A are H1N1 and H3N2. A swine-origin influenza A (H1N1) virus outbreak in 2009 reinforced that interspecies transmission of human and animal influenza A viruses and subsequent reassortment are possible and occur, resulting in new viruses that may spread to humans and even easily between humans. Human infections with variant influenza viruses most commonly happen in people with exposure to infected pigs. A subtype of avian influenza A that emerged in Asia (H5N1) historically produced infrequent but often fatal human illness. In 2024, human cases of H5N1 have been linked to outbreaks among US dairy cows and poultry.