There are three types of influenza virus: A, B and C. Influenza A viruses are found in humans and animals, whereas B and C are found only in humans. The C type virus is rare and usually causes a very mild disease and therefore is not covered by currently available vaccines. The B type virus is often limited to local outbreaks (20% of influenza cases) and their surface proteins are considered to be relatively stable.
Influenza type A viruses are the most common and are considered to be the major cause for widespread epidemics and pandemics. This is due to the frequent changes in their surface proteins which the human immune system recognizes as foreign antigens.
Minor changes in the surface proteins of the virus are termed "antigenic drifts", while major changes in one or both of the surface proteins are known as "antigenic shifts". The greater the change in the surface proteins is, the less effective the body's immune defense (resulting from previous influenza infections) and current vaccines.
Influenza viruses are species dependent (bird flu, swine flu, equine flu etc.) while the migratory birds serve as the reservoir for influenza strains. Antigenic drifts and shifts trigger seasonal epidemics or even a worldwide pandemic of influenza, such as occurred in 1918, 1957, 1968, 1976, 1977 and 1997 (Spanish, Asian, Hong Kong, Swine, Russian and Avian Flu, respectively). Pandemics happen every 10-30 years following a major antigenic shift resulting from a cross species infection.
Structure of the Influenza Virus
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