An outbreak of swine flu in Mexico has raised concerns worldwide that the disease could be emerging as a global pandemic. On April 26, 2009, American officials declared a public health emergency after 20 cases of swine flu were confirmed in the United States; by the next day, the number had doubled.

The virus in the American cases looked identical to the A (H1N1) swine flu in Mexico that is believed to have killed 149 people and sickened about 1,600. Health officials in the United States and at the World Health Organization urged the public not to panic, noting that the cases confirmed outside of Mexico had been mild, and that the virulence of the virus remained unknown.

Still, they urged Americans to forego nonessential travel to Mexico, where many schools and public venues had been shut. On April 27, the European Union's health commissioner urged Europeans to avoid nonessential travel to the United States or Mexico. The acting director of the Centers for Disease Control and Prevention, Dr. Richard Besser, responded that the advisory was unwarranted.

Officials said that 28 of the 40 confirmed cases in the United States were diagnosed in New York City, all among students at St. Francis Preparatory School in Fresh Meadows, Queens. Officials said they had also confirmed cases in California, Kansas, Texas and Ohio. Diagnoses have also been made in Canada, Spain, Scotland and New Zealand.

Mexican officials said they had traced the origins of the outbreak to a rural area known as La Gloria in the southeastern state of Veracruz, the site of several major pig farms.