ABSTRACT
John Snow was the father of the doctrine of specificity of infection in England, as Bretonneau was in France. Snow's marvellous insight into the true causation of cholera and typhoid fever has been brought out in Evolution of Preventive Medicine. Two methods of advance in knowledge of the nature of disease emerged, such as the experiential and the experimental. The stage of evolution in which study of the natural history of disease was the chief means of determining its nature and causation was followed by the sedulous study of diseased organs in autopsies of which Morgagni's work in 1760 is outstanding. The conception of unification in tuberculosis was one of the earliest approaches to knowledge of specificity of disease, and around the conception of specificity has centered the struggle between different views of causation which in some measure still continues. The separation of disease from disease continued steadily as an indispensable part of progress.
