The aim of preventive medicine is the absence of disease, either by preventing the occurrence of a disease or by halting a disease and averting resulting complications after its onset. Preventive medicine can be practiced by governmental agencies, primary care physicians and the individual himself.

From these definitions it is evident that preventive medicine depends on knowing how disease begins and how the individual proceeds through the normal to the diseased state. Therefore preventive medicine demands new facts and the efficient application of known ones, both of which are epidemiologic processes. Epidemiology is the basic science of preventive medicine. The word epidemiology is derived from the Greek words for "upon" and "people". An epidemic is something that happens to a number, of people; epidemiology is the study of the things that happened, whether it is a disease or another factor. The object of epidemiological study is to establish the cause of a condition or the association of a condition with particular characteristics of the affected individuals or their environment.

Epidemiology can find causes, show priorities and evaluate activities. Epidemiologic research is essential to preventive medicine, since one cannot deal with an end product like disease while ignoring its causation. Cancer is probably preventable but at present we do not know enough to prevent it. We have to find its cause, yet most research efforts in cancer are concerned with cure rather than prevention. Historically this seems a poor bet, since most of the diseases which used to plague society have been removed by prevention, not cure -- for example, smallpox. A safe cigarette appears a better target than a cure for lung cancer. Minimizing the effects of any disease by treatment seems less attractive than preventing its occurrence in the first place.