During the spring of 2009, a novel influenza A (H1N1) virus of swine origin caused human infection and acute respiratory illness in Mexico.1,2 After initially spreading among persons in the United States and Canada,3,4 the virus spread globally, resulting in the first influenza pandemic since 1968 with circulation outside the usual influenza season in the Northern Hemisphere (see the Supplementary Appendix, available with the full text of this article at NEJM.org). As of March 2010, almost all countries had reported cases, and more than 17,700 deaths among laboratory-confirmed cases had been reported to the World Health Organization (WHO).5 The number of laboratory-confirmed cases significantly underestimates the pandemic's impact. In the United States, an estimated 59 million illnesses, 265,000 hospitalizations, and 12,000 deaths had been caused by the 2009 H1N1 virus as of mid-February 2010.6 This article reviews virologic, epidemiologic, and clinical data on 2009 H1N1 virus infections and summarizes key issues for clinicians worldwide.