On 17 April 2009, the Centers for Disease Control and Prevention reported that a girl, age 9 years, and a boy, age 10 years, who had flu-like symptoms and lived in adjacent counties in California, had tested positive for an H1N1 “swine” influenza virus (1). Mexico City had reported influenza-like illnesses on 18 March. By 13 May 2009, the strain—which seems to be a reassortant virus (Figure) with nucleotide sequences derived from swine, avian, and human viruses (2)—was confirmed in patients from Mexico, the United States, Canada, Spain, the United Kingdom, and at least 28 other countries. The virus derives the name “swine” from the animal reservoir thought to be responsible for initiating the outbreak in humans. An animal source was not required for subsequent infection because direct human-to-human transmission had become possible.