A commentary on the mechanical analogue to the Reed-Frost epidemic model.

Recent years have witnessed a considerable increase in the development and use of mathematical and computer-based models in infectious disease epidemiology. As a measure of this increase, Bailey (1) finds that the literature in this field has grown at a greater-than-exponential rate over the past two decades/ilthough one may suspect that some of the published models have greater value as exercises in mathematics than as contributions to "real disease" epidemiology, this is certainly not true of all of them; and many examples can be cited of major contributions of mathematical models in revealing relationships underlying complex epidemiologic phenomena, in hypothesis testing, and in the