Cambodian women wear masks as they walk in a market in Phnom Penh in Oct. 2009. That month a second Cambodian died from swine flu, health officials said. Tang Chhin Sothy/AFP/Getty Images hide caption

itoggle caption Tang Chhin Sothy/AFP/Getty Images

Students at a University of London class in Mexico City wear masks to protect them against swine flu in May 2009. High schools and universities closed by the pandemic had just reopened across Mexico. Brennan Linsley/AP hide caption

itoggle caption Brennan Linsley/AP