Update: influenza activity - United States, September 29-December 7, 2013.

CDC collects, compiles, and analyzes data on influenza activity year-round in the United States (http://www.cdc.gov/flu/weekly/fluactivitysurv.htm). The influenza season generally begins in the fall and continues through the winter and spring months; however, the timing and severity of circulating influenza viruses can vary by geographic location and season. Influenza activity in the United States continued to increase from mid-November through the beginning of December. This report summarizes U.S. influenza activity* during September 29-December 7, 2013.