Epidemiology began as an outgrowth of medicine. Many of its earliest practitioners were clinicians who recognized that poor health is often closely related to a patient's environment and that observations and interventions in groups are directly relevant to treatment and prevention of disease in individual persons. For example, the classic studies that demonstrated a causal link between contaminated drinking water and cholera emanated from Snow's belief that quantitative assessment in the community would provide the basis for prevention of illness in individual persons (1). Jenner's historic demonstration of the value of cowpox immunization in preventing smallpox was also rooted in the challenges he faced in his clinical practice (1). During the first half of the 20th century, a number of epidemiologists focused their research on topics directly relevant to clinicians. For example, Frost's observations on the transmission of tuberculosis were of great value in disease surveillance and recognition of new cases (2). However, these and similar findings by the leading epidemiologists of the day were at the periphery rather than the heart of clinical thinking and practice. As the discipline of epidemiology evolved, it became more technical and less clinically oriented. In parallel, the focus of research in medicine began to shift from the applied to the bench sciences. In part, this shift resulted from an increased capacity to conduct bench research and an expectation that enhanced understanding of basic biology would inexorably lead to improvements in the ability to cure disease. In the