Health workers fill in forms for pedestrians crossing to the U.S. at the Puerta Mexico international border crossing bridge in the city of Matamoros April 30, 2009. U.S. and Mexican officials stepped up their checks of people crossing through the border on Thursday to screen travellers for signs of the deadly influenza A (H1N1), formerly referred to as swine flu, that killed up to 176 people in Mexico. Mexican doctors wearing blue surgical masks probed foot passengers on border bridges into Texas and California and looked for those with fever or coughing, while the U.S. customs agents were also on alert. REUTERS/Tomas Bravo (MEXICO HEALTH)