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He grasped germ theory. Franklin was among the first to suggest that colds and flu spread not by cold air, as was thought at the time, but through “contagion” unrelated to temperature. “Traveling in our severe winters, I have often suffered cold sometimes to the extremity only short of freezing but this did not make me catch cold,” he wrote Philadelphia physician Benjamin Rush in 1773. “People often catch cold from one another when shut up together in close rooms, coaches, etc., and when sitting near and conversing so as to breathe in each other’s transpiration.” To avoid colds, he recommended getting as much fresh air as possible. All his life, Franklin ventilated his residences, especially his bedroom, even in winter. And at every opportunity he shed his clothes. (The Granger Collection, New York)