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Disease

  Classification

The first flu virus was identified in the 1930's. Influenza viruses belong to the group of RNA orthomyxoviruses. They are classified as follows:

  • Type A

    Type A is the most common virus type and has been associated with the occurrence of extensive epidemics. This virus type causes respiratory illness almost every winter having a huge impact on public health.

    Influenza type A viruses are divided into subtypes, based on direct antigenic divergence of the two principal surface glycoproteins known as hemagglutinin (H) and neuraminidase (N). Mutations of H and N are known as "antigenic drifts", leading to gradual and minor changes in the viral antigenic proteins over time. This process is continuous for both influenza A and B viruses. Antigenic drifts may result in epidemics.

    This specific behavior of influenza viruses accounts for most of the changes in the viruses from one season to another, enabling them to evade your immune system. Therefore, you remain susceptible to influenza virus infection throughout your life. If your immune system is functioning properly, you will develop antibodies against the viruses you have been infected with. As the virus changes, your "old" antibodies remaining from previous infections are unable to recognize the viruses. This absence of immunity against the "new" virus means that reinfection can occur. Therefore yearly immunization is necessary.

    Another kind of mutation of influenza viruses, known as "antigenic shift", is caused by an abrupt alteration in the H and/or the N proteins. New subtypes of the virus may emerge as a result of entire gene segment exchanges between human and animal viruses (usually of avian origin). Antigenic shift has occurred only with influenza A viruses, resulting in pandemics and affecting large portions of the population, if the virus is efficiently transmitted from person to person. The last major antigenic shift occurred in 1968 when H3N2 (Hong Kong) influenza suddenly appeared. It completely replaced the type A strain H2N2 that had circulated throughout the world for the prior 10 years.


  • Type B

    Influenza type B usually causes mild respiratory disease, but can also be responsible for significant morbidity and mortality during epidemics. This occurs mostly in 3- to 5-year cycles. Influenza Type B viruses mutate only by the more gradual process of antigenic drift and are largely restricted to humans. Currently, only a single serologic type of influenza B virus is circulating around the world.


  • Type C

    Influenza type C has never been connected with a large epidemic. It causes a very mild respiratory disease or no symptoms at all.





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