epidemiology

Epidemiology is the science that studies the distribution and determinants of health-related states or events in populations, and the application of this knowledge to improve health. It seeks to describe patterns by time, place, and person and to identify factors that influence those patterns.

Key measures include incidence, prevalence, mortality, and case-fatality rates, as well as measures of association such as relative risk and odds ratio. Epidemiology also addresses concepts of exposure, risk factors, confounding, bias, and causality, using frameworks to evaluate whether associations may be causal.

Studying epidemiology involves a range of methods. Descriptive or exploratory studies summarize health information, while analytic studies test hypotheses using cohort, case-control, or cross-sectional designs. Randomized controlled trials provide experimental evidence when feasible. Data come from surveillance systems, vital records, disease registries, health surveys, and electronic health records. Epidemiology also encompasses outbreak investigation and the evaluation of public health interventions.

Subfields include infectious disease epidemiology, chronic disease epidemiology, social epidemiology, and genetic epidemiology. The field has a long history, with John Snow's investigation of the cholera outbreak often cited as a turning point. Today, epidemiology informs policy and practice, guiding vaccination programs, screening strategies, safety regulations, and health promotion efforts, while emphasizing ethical considerations and data quality.