Update: influenza activity--United States and worldwide, May 22-September 3, 2011.

During May 22-September 3, 2011, the United States experienced low levels of influenza activity; 2009 influenza A (H1N1), influenza A (H3N2), and influenza B viruses were detected worldwide and identified sporadically in the United States. Typical seasonal patterns of influenza activity occurred in the Southern Hemisphere. This report summarizes influenza activity in the United States and worldwide since the last update.