25_EVOW_CH12.jpgInfluenza A experiences another type of mutation called antigenic shift that results in a new subtype of the virus. Antigenic shift is a sudden change in antigenicity caused by the recombination of the influenza genome, which can occur when a cell becomes simultaneously infected by two different strains of type A influenza. The unusually broad range of hosts susceptible to influenza A appears to increase the likelihood that this event will occur. In particular, the mixing of strains that can infect birds, pigs, and humans is thought to be responsible for most antigenic shifts. Notably, in some parts of the world, humans live in close proximity to both swine and fowl, so that human strains and bird strains, may readily infect a pig at the same time, resulting in a unique virus. New subtypes of influenza A develop abruptly and unpredictably so that scientists are unable to prepare vaccines in advance that are effective against them. Consequently, the emergence of a new subtype of the virus can cause a global pandemic in a very short amount of time.

Molecular Expressions 2005, cell biology and microscopy structure and functions of cells and viruses, Tampa, Florida, viewed 27 October 2009, <http://micro.magnet.fsu.edu/cells/viruses/influenzavirus.html>
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By: Bishoy Aiad