MEXICO CITY - MAY 01: People line up wearing surgical masks, to prevent contracting the swine flu, as they wait to see a doctor at a mobile health clinic on May 1, 2009 in Mexico City, Mexico. Cases of swine flu, the strain known as H1N1 virus, have been confirmed in nine countries, including Mexico, where at least 2,400 are believed to be infected, with the number of deaths attributed to the virus believed to be more than 150. The World Health Organization today raised the threat level to 'phase 5,' indicating a pandemic is imminent. (Photo by Joe Raedle/Getty Images)