Epidemiology is today a basic discipline for studying the occurrence and distribution of ill health in populations and elucidating the factors involved. Examples are the unraveling of the etiology of pellagra, the connection of congenital defects in the fetus with the occurrence of German measles in the mother, as well as the linking of lung cancer and cigarette smoking. The term" epidemiology" did not come into use until the latter half of the 19th century. The Epidemiological Society of London was organized on July 30, 1850, and the first book apparently to use the word in its title was John Parkin's" Epidemiology; or the Remote Cause of Epidemic Diseases in the Animal and in the Vegetable Creation"(1873). None-theless, neither the problems central to epidemiology at that timenor its methods were totally new. As it appeared in the 19th century, epidemiology was in fact the product ofa long tradition extending back to antiquity, and had been develop-ing for several centuries along lines which were to give it its specific character. Epidemiology in its literal derivation means the doctrine or science of epidemics. Indeed, the origins of the discipline are to be found in the narrative descriptions and historical accounts of epidemics. The initial focus of epidemiology was on the investigation of epidemic diseases in order to promote their control. Involved was the notion that such conditions were due to the deleterious effects of environmental influences on the human body. But on the nature of these influences and their mode of action there was profound disagreement, and it was largelythrough the increasingly sophisticated investigation of epidemics that epidemiology developed to its present status as a scientific discipline. During thepast fifty years the scope of epidemiology has expanded greatly so that it is no longer concerned alone with communicable diseases but with the understanding of all factors related to the oc-currence of disease among groups of people. It involves the study of populations, the well and the sick, their relationships to each other and to their environment. Epidemiology encompasses all diseases, acute and chronic, communicable and non-communicable, epidemic and endemic. Finally, the application of epidemiolog-