The detection and spread of pandemic 2009 H1N1 influenza in the United States led to a complex and multi-faceted response by the public health system that lasted more than a year. When the first domestic case of the virus was detected in California on April 15, 2009, and a second, unrelated case was identified more than 130 miles away in the same state on April 17, 2009, the unique combination of influenza virus genes in addition to its emergence and rapid (...) spread at the end of the typical Northern Hemisphere influenza season suggested the potential for a high morbidity, high mortality event. In response, federal, state, and local public health officials conducted epidemiologic investigations with federal and state laboratory support to help to determine the scope of the H1N1 pandemic. On April 26, the Secretary of the U.S. Department of Health and Human Services declared a public health emergency that was renewed through June 23, 2010. The pandemic that ensued tested virtually every aspect of U.S. public health preparedness and response systems, from laboratory capabilities and capacities to social distancing plans. (shrink)