What is the historical incidence of deadly influenza pandemics?

Updated: Aug 07, 2020
  • Author: Hien H Nguyen, MD, MS; Chief Editor: Michael Stuart Bronze, MD  more...
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Although the seasonal strains of influenza virus that circulate in the annual influenza cycle constitute a substantial public health concern, far more lethal influenza strains than these have emerged periodically. These deadly strains produced 3 global pandemics in the last century, the worst of which occurred in 1918. Called the Spanish flu (though cases appeared earlier in the United States and elsewhere in Europe), this pandemic killed an estimated 20-50 million persons, with 549,000 deaths in the United States alone. [11]

Besides humans, influenza also infects a variety of animal species. Some of these influenza strains are species-specific, but new strains may spread from other animals to humans (see Pathophysiology). The term avian influenza, in this context, refers to zoonotic human infection with an influenza strain that primarily affects birds. Swine influenza refers to infections from strains derived from pigs. The 2009 influenza pandemic was a recombinant influenza involving a mix of swine, avian, and human gene segments (see H1N1 Influenza [Swine Flu]).

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