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Influenza virus is a single-stranded RNA virus with a segmented genome. The virus is medium- size, consisting of spherical and filamentous particles. Both types A and B have 8 segments, while type C has 7 segments. It has an envelope with characteristic structures projecting from its surface. These are hemagglutinin (HA) and neuraminidase (NA), which are used for classification of influenza A viruses.