Influenza (flu) is a contagious respiratory illness caused by influenza viruses. Some people, including the elderly, infants and pregnant women at high risk for serious flu complications. The flu is a tricky virus that mutates every year. While that may make it more challenging to predict the most infectious strands, healthcare researchers use flu data from the other side of the world, where flu season begins earlier than the US. That information is what helps the United States select which strands to use to create the most effective vaccine.
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The best way to prevent flu is by getting vaccinated each year: