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The most virulent form of the flu virus (seen here under an electron microscope in a tissue sample), known as type A influenza, consists of nothing more than 8 strands of RNA, enclosed in protein capsules, surrounded by a globular envelope of proteins.
Like all viruses, influenza needs to take over a host cell in order to reproduce. Two proteins -- hemagglutinin, or HA, and neuraminidase, or NA -- found on the surface of the virus bind to proteins on human and animal cells and act as the chemical "keys" letting influenza enter and take over those cells.
Researchers have identified 16 major types of HA protein and 9 types of NA protein circulating among flu viruses in nature, and refer to influenza subtypes by which types of proteins they include: thus a flu virus whose envelope includes type 2 HA protein and type 3 NA protein is known as "H2N3."
Two other types of influenza, B and C, exist, but do not exhibit the same sort of variation.
Credit: Centers for Disease Control/C. Goldsmith, J. Katz, and S. Zaki
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Interactive Map
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Interactive Challenge
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