Update: influenza activity--United States, August 30, 2009-January 9, 2010.

The emergence and spread of the 2009 pandemic influenza A (H1N1) virus (2009 H1N1) resulted in extraordinary influenza activity in the United States throughout the summer and fall months of 2009. During this period, influenza activity reached its highest level in the week ending October 24, 2009, with 49 of 50 states reporting geographically widespread disease. As of January 9, 2010, overall influenza activity had declined substantially. Since April 2009, the dominant circulating influenza virus in the United States has been 2009 H1N1. This report summarizes U.S. influenza activity from August 30, 2009, through January 9, 2010.