Epidemiology

Epidemiology is the science of how health and disease are spread across populations, what determines those patterns, and how that knowledge can be used to control health problems. Distribution means who is affected, where and when; determinants are the causes and risk factors behind the patterns. Its unit of study is the population rather than the individual patient, which sets the epidemiologist apart from the clinician at the bedside.

The field grew out of the study of infectious outbreaks but now covers chronic diseases, injuries, behaviors such as smoking, environmental exposures and genetic risk. Descriptive epidemiology counts cases and describes them by time, place and person, using measures such as incidence and prevalence. Analytic epidemiology asks why, comparing groups in cohort studies, case-control studies and randomized trials to test whether an exposure raises or lowers risk.

Epidemiology underpins public health: it detects outbreaks, tracks trends, identifies risk factors, guides vaccination and screening, and checks whether interventions work. Classic examples include John Snow's mapping of cholera deaths around a London water pump in 1854 and the studies that tied smoking to lung cancer.