Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems.
Several terms used in this definition require discussion. “Study” encompasses (a) observations in public health practice, e.g. to investigate outbreaks of food poisoning, identify the source of an epidemic, trace contacts of sexually transmitted diseases; (b) hypothesis-testing observational studies using cross-sectional, cohort and casecontrol methods; and (c) experiments such as randomized controlled trials, in which there is intervention as well as observation, usually with the aim of evaluating a preventive, diagnostic or therapeutic regimen or procedure. “Distribution” refers to time sequences, places of occurrence, and persons affected. “Determinants” are all the physical, biological and behavioral factors that can influence health. “Health-related states or events” include diseases, injuries, deaths, behaviors such as tobacco addiction, and reactions to preventive, diagnostic or therapeutic regimens or procedures.