At its core, epidemiology is concerned with changes in health and disease. The discipline requires counts and measures: of births, health disorders, and deaths, and in order to make sense of these counts it requires a population base defined by place and time. Epidemiology relies on closely defined concepts of cause - experimental or observational - of the physical or social environment, or in the laboratory. Epidemiologists are guided by these concepts, and have often contributed to their development. Because the disciplinary focus is on health and disease in populations, epidemiology has...