Since the beginning of this century there has been a growing interest in the study of the epidemiology and population dynamics of infectious disease agents. Mathematical and statistical methods have played an important role in the development of this field and a large, and sophisticated, literature exists which is concerned with the theory of epidemiological processes in popu lations and the dynamics of epidemie and endemie disease phenomena. Much ofthis literature is, however, rather formal and abstract in character, and the field has tended to become rather detached from its empirical base....