Influenza, or the flu, sickens millions of people and is responsible for up to 500,000 deaths worldwide each year. New influenza viruses are produced by mutation or by reassortment-a process in which two similar viruses infect the same cell and their genetic material mixes. New flu vaccines are formulated every year based on the strains observed in the previous year and on expert opinion about which strains are expected to dominate worldwide in the next flu season.
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