Seasonal influenza, a contagious respiratory illness, strikes every year and affects 5% to 20% 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.