In April 2009, a novel influenza virus began to spread around the world. The World Health Organization (WHO) refers to the virus as Influenza A(H1N1). The U.S. Centers for Disease Control and Prevention (CDC) and other Administration officials refer to it as 2009 H1N1 flu. Throughout this report, the virus is referred to as H1N1. Although H1N1 does not appear to be as lethal as H5N1 avian influenza, which reemerged in 2005, the virus is slightly more lethal than seasonal flu and it continues to spread.