It's part of American folk lore: The man who saves the day and rides off into the sunset without anybody knowing his name. Sometimes he doesn't even leave a silver bullet. Dr. Alexander Langmuir, the father of infectious disease epidemiology, was such a man. "He had an absolutely profound impact on saving lives," said Alan Summer, Dean of the Johns Hopkins University School of Hygiene and Public Health and a former student of Langmuir's. "If one wants to include important roles he played, the people he trained and what they've gone on to do, a figure in the millions (of...