Public health has traditionally been concerned with the primary prevention of disease, either by eliminating its causes or by enhancing host resistance. Clean water supplies, efficient sewage and rubbish disposal, and refrigeration of food have largely eliminated the micro-organisms responsible for a wide range of infectious diseases, while anti-smoking campaigns have reduced mortality from cancer of the lung and coronary heart disease. Immunisation programmes have had a similarly dramatic effect on the incidence of many bacterial and viral diseases. Indeed, they have completely eliminated smallpox, and the polio virus seems likely to follow it into oblivion.