Epidemiology is the study of disease distribution and of the determinants of health and disease risk in human populations. A principal aim of epidemiological studies is to determine how and why disease rates vary among populations and what factors are associated with increased risk of disease.  Another major objective is to quantify the magnitude of the relationship between risk factors and disease. 

Epidemiologists use standardized methods and tools to describe disease occurrence in a given population, to identify populations at high risk, and finally, to evaluate causal factors.  Epidemiologists interpret results within the context of available information and data in order to critically assess the weight of the entire body of evidence.  Epidemiologic data provide a quantitative foundation for public health and clinical research, as well as a basis for preventive approaches in medicine and public health.