In the United States, flu season occurs in the fall and winter, according to the Centers for Disease Control and Prevention. While influenza viruses circulate year-round, most of the time flu activity peaks between December and February, but activity can last as late as May. Flu seasons in Nevada are consequently defined as beginning in the 40th week of a given year (usually in early October) and ending in the 20th week of the following year (usually in late May).