The study of disease as it affects groups of people. Originating in the study of epidemics of diseases like CHOLERA, PLAGUE and SMALLPOX, epidemiology is an important discipline which contributes to the control not only of infectious diseases but also of conditions such as heart disease and cancer. Their distributions in populations can provide important pointers to possible causes. The relation between the environment and disease is an essential part of epidemiology. (See ENVIRONMENT AND HEALTH; PUBLIC HEALTH.)