Seasonal influenza, a contagious respiratory illness, strikes every year—affecting 5 to 20 percent of the U.S. population annually. In the U.S. alone, annual deaths caused by flu range from 3,300 to 48,600 people, and more than 200,000 are hospitalized each year from flu-related complications. The best way to prevent influenza is by getting a flu vaccination every year.
There are three types of influenza viruses, classified as A, B, or C, based on their protein composition. Public health experts are most concerned with type A. Type A viruses are subdivided into groups based on two surface proteins: hemagglutinin (HA) and neuraminidase (NA), which are represented as H1 through H16 and N1 through N9.
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