Update: influenza activity--United States, September 30-November 24, 2012.

CDC collects, compiles, and analyzes data on influenza activity year-round in the United States. 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 viruses were detected in the United States throughout the summer months, and activity increased steadily during October and November. Most influenza viruses characterized thus far this season are well matched to the 2012-13 vaccine viruses. This report summarizes U.S. influenza activity* during September 30-November 24, 2012.†