Definition: The family of viruses that contains the three genera of influenza viruses, types A and B, C, and Thogoto-like viruses. Virions are roughly spheric or filamentous, and the former (the more common form) are 80–120 mm in diameter and ether-sensitive; envelopes are studded with surface projections; nucleocapsids are of helical symmetry, 6–9 nm in diameter, and contain single-stranded, segmented RNA. The nucleoprotein antigen of each type of virus is common to all strains of the type but is distinct from those of the other types; the mosaic of surface antigens varies from strain to strain. Nucleocapsids seem to be formed in the nuclei of infected cells, hemagglutinin, and neuraminidase in the cytoplasm; virus maturation occurs during budding of the cell membrane. Influenza virus types A and B are subject to mutation resulting in epidemics. Influenza virus C differs from types A and B (lacks neuraminidase) and belongs to a separate genus.
See Also: Influenza virus
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