In July 1917, when German forces first used mustard gas (sulfur mustard) as a chemical warfare agent against British troops in Ypres, Belgium, no one would have predicted that any good would come of it. The first hint came in 1919, when E.B. and H.D. Krumbhaar reported that victims of mustard gas poisoning had low white blood cell counts and atrophy of the bone marrow and lymphatic tissues. The...