The inference that disease is practically coeval with the existence of life seems justified by the testimony of prehistoric animal and human remains. This evidence supports the speculation that even the earliest forms of life must have been subject to competitive, toxic, and nutritional stresses which they were not always able to withstand. Surely, for practical purposes, we may assume that disease has always been among us human beings. It thus takes its place among natural phenomena like sunlight and gravitation.

For a long time man refused to regard disease as a natural phenomenon. It presented such an affront to his assumed dominance over nature that he could conceive of it only as a visitation from supernatural sources. Only with the development of natural science within the past few hundred years has it become possible definitely to classify disease among the phenomena which are subject to natural law. In ways similar to those by which man has learned to employ gravitation and sunlight to his advantage, he is learning how to avoid the ravages of disease. These methods consist essentially of first learning by research the laws which control the operation of these phenomena, and then, by suitable conduct in accord with these laws, of securing the desired ends.

History records attempts at very early periods to escape disease both by individual and collective effort. The latter represents the beginnings of public health work. In the absence of more than the meagerest knowledge of the nature or laws of disease, these efforts were seldom very effectual, never perhaps economical, and at times possibly even deleterious. At the present time, in instances where our knowledge is scanty, our efforts must be characterized by similar shortcomings.

Health is life without disease. Recently propagandists have attempted to distinguish a condition of positive health in which the possessor is not only not sick, but is super-healthy. It is only a question of where we draw the line. A condition of maximum ability to perform useful functions combined with adaptation to and enjoyment of environment, is certainly one of health, and anything less than this is, in a sense, disease.

Public health work is collective effort to secure a condition of health for all citizens of the area in which it is undertaken. Judged by the progress which public health work has made since its modern inception, it is one of the most rapidly advancing and profitable of human activities. The very fact that it has been highly successful in certain directions has removed the evidence of its achievements from the observation of the average person. Those of our citizens who are without a historical background do not realize that but for the public health work which has been accomplished they might, at the present time, be going through epidemics of Asiatic cholera, bubonic plague, or yellow fever. These and similar conquests which are really revolutionary in their scope are what cheer the workers in this field to renewed effort, and justify the support which the public none too generously gives to the continuation and expansion of the work.

A distinction must be made between personal hygiene and public health work. There are many persons who appear to think that if they individually carry out an ideal set of rules, they will be able to avoid illness and become or remain healthy. Undoubtedly they would further assume that if all individuals did the same, disease would disappear. This of course is an incorrect generalization based upon a part truth. Hygienic living by individuals is highly desirable, and tends to reduce their liability to illness, and their menace to others. It is one of the objectives of public health work to secure the observance of personal health practices by individuals. On the other hand, there are numerous factors in the induction and transfer of disease which cannot be dealt with by individual effort, at least not practically or economically. The man who lives in a community where malaria is prevalent may observe hygienic rules, dose himself with quinine, and stay behind screens after dusk, and he may thus for a period, and at considerable sacrifice, avoid infection; but in the long run he is likely to overlook some loophole in the scheme and come down with chills and fever. On the other hand, by well-directed community effort it is often possible to eradicate the infection from the area, so that no person living there, even if his personal habits were careless, could acquire the infection. Against typhoid fever again, public health measures, such as the providing of safe water and milk supplies and sewage disposal, insure a cheaper, more certain, and more general protection than could ever be expected from the exercise of individual precautions by members of the population concerned. Again, in the case of some pestilences such as yellow fever and bubonic plague, the individual could hardly secure immunity except at the sacrifice of nearly all other activity, while public measures can be counted upon for speedy control of epidemics.

We have in the ancient Hebrew sanitary measures commonly attributed to Moses an excellent example of rules for personal health which, while limited in scope and often redundant and confused with religious observances, undoubtedly made for better health of those observing them, under the conditions then obtaining. Is this public health or personal hygiene? In so far as it represents an attempt by authority to influence the members of the population to avoid disease by personal practices, it is public health work, but the practices themselves will be found to be very largely of the nature of personal health habits.

In contrast to this may be cited the huge engineering undertakings of the ancient Romans, which had a sanitary and salutary effect, whether we may be able to trace the intent or not. The aqueducts, bringing presumably purer water from relatively uncontaminated sources, and the cloacae in turn removing the dangerous wastes, were essentially public health works.