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Mexico City, the world's third largest metropolis, was effectively shut down when a new and deadly virus, swine flu appeared. Soon the virus started to spread and was seen as a massive threat to global health. Experts feared millions of people could become infected and many countries began screening airline passengers for symptoms and suspending flights to Mexico.

Photo: People wear surgical masks as they ride the subway in Mexico City (Ronaldo Schemidt/AFP/Getty Images)