John Brownlee and the Measurement of Infectiousness: An Historical Study in Epidemic Theory

Abstract

SUMMARY THE development of the germ theory, during the course of the nineteenth century, was a pivotal event in the history of medicine. Its impact was enormous, leading all of the biomedical sciences away from previously held notions of miasmas and humours, towards more rigorous concepts of specific aetiologies and appropriate preventive and therapeutic measures. Though the discoveries of Pasteur and of Koch stand out in any history of the germ theory, its develop- ment involved major contributions by many other workers. Included in the effort were not only microbiologists, but also a number of epidemiologists, notably Panum and Snow, whose detailed studies of population patterns of disease seriously challenged the notion of miasmas and resurrected Fracastoro's idea of a contagium vivum. It is interesting to note that virtually all of the builders of this germ were practical men. Though there were obvious theoretical implications to their work, they were in a very real sense not theorists; and their immediate interests were directed towards practical solutions of specific disease problems. Thus Panum advocated isolation and quarantine against measles, Snow removed the handle of the pump incriminated in cholera transmission, and Pasteur repeatedly applied his researches to the development of vaccines and other control measures. The pure theorists came later, those who generalized and abstracted the information about infectious in order to derive global explanations for the patterns of disease in time, space and populations. There were many problems which lent themselves to such theorizing, which could indeed hardly be approached otherwise: the duration and frequency of epidemics, the relationship between population size and disease pattern, the spatial spread of disease, the age distribution of disease, and so forth. And essential to the building of this body of theory was the development of an appropriate quantitative measure for infectiousness, or infectivity, the fundamental property of those germs which had been found responsible for disease. But this turned out to be a surprisingly difficult problem, whose solution required many years. Furthermore, it raised ancillary problems which had an important influence upon much microbiological and epidemiological work for almost half a century. It is the intent of this paper to discuss the evolution of this quantitative concept of infectivity as it is reflected in the theoretical epidemiology literature. Because he was the first to formulate