## TL;DR
Epidemiology is the cornerstone of public health — the systematic study of disease distribution, determinants, and control in populations.

## Core Explanation
The epidemiological triad (agent, host, environment) models disease causation. Study designs: descriptive (case reports, surveys), analytical (cohort, case-control), experimental (RCTs). Measures of association — relative risk, odds ratio — quantify exposure-outcome relationships.

## Detailed Analysis
Key metrics: incidence (new cases/period), prevalence (existing cases). Confounding, bias, and effect modification are primary validity threats. Bradford Hill's criteria (1965) provide a framework for assessing causation: temporality, dose-response, consistency, biological plausibility.

## Further Reading
- WHO Global Health Observatory
- CDC Morbidity and Mortality Weekly Report
- The Lancet Infectious Diseases