The Centers for Disease Control defines the reporting period for the flu season as 52 weeks, starting at the 40th week of one year through the 39th week of the next. The visualizations below track the frequency of flu cases over time per U.S. state.

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Flu activity level (2019-2020) 0 10 The 10 activity levels compare the current week’s percent of doctor visits due to flu-like symptoms to the weekly average percent outside of flu season. A level of 1 is a lower percentage than the out-of-flu season average, with a level of 2 about equal. Levels from 3 to 10 are progressively higher.