PROBLEM NUMBER 1: THE SHIFTING ECOLOGY OF DISEASE

We all know that human disease is a constant and natural expression of conflicting forces and agents. As biological phenomena, diseases are never static. Influenced by physical and social agents, sickness shifts constantly in specific ecological settings. In infectious diseases, for example, constant readjustments within changing physical environments occur between pathogenic agents such as micro-organisms and viruses and their human hosts. As do other complex organisms, humans display a series of adaptive mechanisms which can both prevent or create illness; a lifetime of such encounters categorized as ‘wear and tear’ phenomena leads to distinctions between chronological and physiological age that are useful in assessing susceptibility to disease and life expectancy.

To understand the broad contours of human disease and periodic epidemiological changes in various parts of the globe – what physicians have called the ‘ebb and flow’ of disease – many historians therefore subscribe to the idea of a shifting ecology of disease. This dynamic concept presupposes complex interactions between both biological and non-biological factors which are ultimately responsible for different and changing patterns of sickness in time and space, exemplified, at least superficially, by McNeill's popular book Plagues and peoples. By highlighting the interconnectedness of various possible factors, including those which could ultimately cause death, the ecological model is quite useful to both historians and demographers.