Counting cases and comparing groups reveals how disease behaves. Epidemiology is the study of the distribution and determinants of health-related states in populations, and the application of that study to control health problems.

Descriptive epidemiology maps who, when, and where; analytic epidemiology tests hypotheses about causes. Cohort, case-control, and randomized designs each contribute evidence. Surveillance systems provide the raw data that detect outbreaks and track trends.

Epidemiologic methods underpin vaccination policy, cancer screening guidelines, and responses to emerging infections. They also evaluate healthcare interventions and social determinants. Clear communication of risk and uncertainty is part of the discipline’s public role.

• Study of disease distribution and causes in populations
• Uses descriptive and analytic methods
• Guides prevention, policy, and outbreak response
• Relies on rigorous study design and surveillance