Throughout human history diseases have affected social and political development. At the height of Athenian culture and power, the plague unraveled Athens' civic and moral fiber. Napoleon's Russian campaign was thwarted more by typhus than by battle. Today, AIDS is devastating the southern part of the African continent. Our understanding of how diseases impact civilizations and of how exposures such as smoking, cholesterol, mammography, and immunizations affect us individually is derived through epidemiology, the study of the causes, transmission, incidence, and prevalence of health and disease in human populations. Epidemiology is also one of the keystones to developing disease-prevention strategies and for health and disease policy, enabling policymakers to understand the potential impact their recommendations may have on populations. Perhaps more important, for curious individuals, a knowledge of epidemiology helps us understand how to assess risk of health hazards as expressed in the media, scientific publications, policy studies, and political debates. How do we know the risk of catching HIV from a single sexual encounter? With all the talk of smallpox as a bioweapon, what is the risk of an adverse reaction to smallpox vaccination? We begin with basic concepts of health and disease definition and distribution. We then discuss disease rates, causation, research and screening methods (cross-sectional, cohort, case-control, and experimental designs), measurement error and bias, and how to critically read the health/medical literature. Throughout these discussions we use case studies in infectious, chronic, molecular/genetic, occupational, and social epidemiology. The social impact of epidemiology is illustrated through the discussion of contemporary health policy issues. Prospective students should be aware that while statistics per se is not an emphasis in this course, there is a need to understand and perform arithmetic calculations.