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Over the years, Robert Daum has learned to respect his adversary. In 1995, he and his co-workers at the University of Chicago children's hospital in Illinois were investigating infections that had affected two dozen children in their emergency department. Three children had fast-moving pneumonia. A fourth had an abscess the size of his fist buried in the muscle of one buttock. In a fifth, the bacterium had infiltrated the bones of one foot. The infections were resistant to many common antibiotics, including methicillin. To Daum's surprise, the culprit was MRSA — methicillin-resistant Staphylococcus aureus — a bacterium that was thought to spread only among hospital inpatients. But none of these kids had been to the hospital for months before becoming...