To Dr. Paul H. DeKruif I am indebted not only for most of the bacteriological and medical material in this tale but equally for his help in the planning of the fable itself⁠—for his realization of the characters as living people, for his philosophy as a scientist. With this acknowledgement I want to record our months of companionship while working on the book, in the United States, in the West Indies, in Panama, in London and Fontainebleau. I wish I could reproduce our talks along the way, and the laboratory afternoons, the restaurants at night, and the deck at dawn as we steamed into tropic ports.