The history of medicine, and especially of preventive medicine, being coterminous with the history of science, it is concerned, in its earlier stages, with various obsessions of the human mind, which successively have been elevated into the position of doctrines or dogmas. An erroneous doctrine strangles investigation and original thought; and the history of medicine is strewn with the wreckage showing unnecessary suffering due to delay in the study of the natural history of disease and of the collateral biological sciences. Of the various obstacles to progress the one which stands out with special prominence is what may be called the Sacerdotal Obsession. Even though risking historical orderliness, the authors illustrate the theological view of disease from modern experience. As generations followed generations, it is not surprising that men began to associate disease and other calamities with the maleficent influence of angry gods, and that priest for long ages were the chief or the sole physicians.