We can never hope to diminish the total number of deaths which will occur in long periods, say two hundred years, but we may rationally try to prolong the average duration of life, to diminish infant mortality, and to secure greater comfort and better health for individuals and for the community at large. The reader must remember that only a mere outline of the subject can be presented here; the details would require several volumes, and the tendency to specialization in this, as in other branches, is so great that it is hardly to be expected that any one man shall have either the theoretical or the practical knowledge necessary for covering the entire field. There are certain things in relation to hygiene which every physician should know; there are many other things with regard to which it is sufficient if he knows where to find full and reliable information when he needs it. With this preface we will pass at once to our subject, which may be conveniently divided as follows: I. Causes of disease, means of discovery, and prevention. II. Personal hygiene in its relations to the practice of medicine. III. Public hygiene in its relations to physicians. I. Causes of Disease, Means of Discovery, and Prevention. Although the origin of disease has from the earliest times been the subject of study by medical men, the physician has not heretofore, usually, been called upon to investigate the causes of disease in particular localities, until the occurrence of sickness in that locality has called attention to the matter. The education of the public as to the importance of sanitary work has, however, recently made great progress, and it is now not unusual to ask the opinion of the family physician as to the healthfulness of a given locality or house. The question may be presented in three different ways: First. In a given case of disease, what is the probable cause? Second. Given the presence of a known or suspected cause of disease, what are the best means of avoiding or destroying it? Third. In the absence of cases of disease, to determine whether causes of disease are probably present, and if so, what causes. The word "cause" is here used in its widest sense, including not only what are commonly called predisposing and exciting causes, but also those conditions which aggravate or continue the disease. These causes may be roughly classed as follows: Heredity; impure air; impure water; climate; habitations; occupation; food; intemperance of various kinds; clothing; errors in exercise; sexual errors; parasites; contagia; expectant attention and other mental causes, including worry, etc. In most cases two or more of these classes of causes are combined in action for the production of a given case or outbreak of disease, and when we refer any disease to a single factor, what is meant usually is, not that this is the sole and exclusive cause, but that it is the most prominent one.