“The timing of flu is very unpredictable and can vary in different parts of the country and from season to season. However, seasonal flu activity can begin as early as October and continue to occur as late as May. Flu activity most commonly peaks in the U.S. between December and February.”

CDC Centers for Disease Control and Prevention (“What should you know for the 2014-2015 influenza season”.  http://www.cdc.gov/flu/about/season/flu-season-2014-2015.htm)