This is a major synthesis of the theory and empirical knowledge about the ecology and epidemiology of infectious diseases in natural, unmanaged, animal and plant populations. Throughout the book a dialogue is developed between the patterns observed in empirical studies of disease in natural populations and the mathematical models used to dissect and examine the observed epidemiological patterns. The book arose from a symposium at the Newton Institute at Cambridge University. It is divided into a number of reviews by experts in various fields and four group reports: two of these synthesize important issues relating to the dynamics of microparasites and macroparasites, while the others discuss spatial patterns in disease dynamics and the evolutionary biology of parasites, pathogens and their hosts.