Policemen walk outside the mayor's office, next to Mexico City's main square Zocalo May 1, 2009. Sidewalks were quiet in central Mexico City, a sprawling metropolis that is home to some 20 million people, after Mexican President Felipe Calderon asked Mexicans to stay home from May 1 to 5 to help slow the spread of influenza A (H1N1), formerly referred to as swine flu. REUTERS/Eliana Aponte (MEXICO HEALTH CITYSCAPE)