We can consider the ecology of disease to be the sum total of all the influences on pathogens and their hosts and, because of the interdependence of the two for disease expression, the internal structures and systems of both that bear upon a disease question. Thus we are clearly considering a structure of relations whose complexity surpasses all comprehension. Such may also be said of a single disease. No one has yet fully defined all that constitutes the expression of even one disease, and such an explanation would be its ecology. To describe the disease ecology of Europe is a task at first so daunting as to admit no possibility. Still, there are a few aspects of the ecology of disease in Europe that can be described, if without claim to completeness or ultimate value, at least with an eye to creating a target for more detailed studies and criticism.

With such cautions in mind, I should like to offer a few general comments on human disease and its expression, and then set forth a very limited number of aspects of the totality of my subject, which I feel can be discussed. For most of the sojourn of humankind on Earth, we have only skeletal remains upon which to build any concept of disease in the past. Even after the advent of agriculture and the earliest civilizations, we have little upon which to develop a coherent view. General trends are perceptible in classical times, and very sketchy numbers can be offered for population in the late medieval period.