Influenza A, B, and C viruses are segmented, single-stranded, negative-sense RNA viruses that have on their outer surface prominent spikes formed by two surface glycoproteins, hemagglutinin (H) and neuraminidase (N). Influenza A and B viruses are major human pathogens, but influenza C virus causes only intermittent mild disease.

  • Influenza A viruses are subtyped by the H and N antigens.
    • Virus attaches to sialic acid cell receptors via the hemagglutinin. Neuraminidase cleaves the virus from the cell membrane to facilitate its release from the cell.
    • Influenza A viruses undergo antigenic drift (modification of immunogenic epitopes, particularly on the H antigen), which counteracts prevailing immunity, and antigenic shift (reassortment of genes among different isolates), which results in major changes in antigens). The viruses have extensive mammalian and avian reservoirs. These features give influenza A virus the ability to cause a worldwide epidemic (pandemic).
  • Influenza is transmitted by small- and large-particle droplets. Virus spread is facilitated by the coughing and sneezing that accompany the illness.

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