While seasonal influenza (flu) viruses are detected year-round in the United States, flu viruses are most common during the fall and winter. The exact timing and duration of flu seasons can vary, but influenza activity often begins to increase in October. Most of the time flu activity peaks between December and February, although activity can last as late as May.

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Photo credit:  Eneas De Troya from Mexico City, México [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)]