Printed in Great Britain John Snow, the Broad Street Pump and Modern Epidemiology

John Snow was a genius in epidemiology. His achievement was to evolve an elegant, internally and externally con-sistent theory which concerned the mechanisms and processes involved in every aspect of the subject he had chosen to study. In order to do this he did not restrict himself to any method. He used all skills available to himself and his colleagues. He published his theory, and practical suggestions for the prevention of cholera arising out of it, both internally in medical meetings and to the medical press, and to the public in the form of pamphlets and in reports addressed to the appropriate authority. This surely should be the objective of all epidemiological work. In a recent paper in this journal on the Broad Street Pump Dr Smith makes a construction based on a theory which has nothing to do with the behaviour of the cholera vibrio or with the people involved.1 It consists of assumptions about the number of persons at risk and about a constant infectivity of the water supply, and it assumes that water was the only medium for the transmission of the disease. These assumptions are purely speculative and are at variance with contem