Update: influenza activity - United States, August 30-October 31, 2009.

The 2009 pandemic influenza A (H1N1) virus emerged in the United States in April 2009 and has since spread worldwide. Influenza activity resulting from this virus occurred throughout the summer and, by late August, activity had begun to increase in the southeastern United States. Since August, activity has increased in all regions of the United States. As of the week ending October 31, nearly all states were reporting widespread disease. Since April 2009, pandemic H1N1 has remained the dominant circulating influenza strain. This report summarizes U.S. influenza activity from August 30, 2009, defined as the beginning of the 2009-10 influenza season, through October 31, 2009.