MEXICO CITY - APRIL 28: Hospital workers walk past the main entrance of the Mexican Institute of Respiratory Illnesses on April 28, 2009 in Mexico City. Concern over the Swine Flu spreads worldwide.
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MEXICO CITY - APRIL 28: Hospital workers walk past the main entrance of the Mexican Institute of Respiratory Illnesses on April 28, 2009 in Mexico City. Concern over the Swine Flu spreads worldwide.