The Germ Theory of disease, which states that illness is caused by germlike substances, was first suggested in the fourth century B.C. by the Greek philosopher Democritus. More than two millennia later, in the latter half of the nineteenth century, the French chemist and microbiologist Louis Pasteur and the German physician Robert Koch were finally able to establish the theory, based on microscopic observation and experimental evidence. For most of the intervening centuries, the concept of Germ Theory was doubted and other ideas about the origin of disease prevailed. Predominant among these was the notion that illness was caused by lethal emanations in the atmosphere, also known as "miasmas." Miasmas might be generated from swamps, from the decomposition of plant and animal matter, from sewage, or from various climatic events. As late as the 1840s, the mainstream medical profession in England was espousing the view...