When bacteriology as applied to medicine began to yield quantitative results, health students began to discriminate regarding filth. The invisible typhoid bacillus concealed in a sparklingly clear well water became vastly more important than the pile of ashes in the back yard. Distinctions were made as to whether nuisances were actual, potential, or negligible health menaces. Economies were thus effected in action, or former duties were turned over to other public agencies. The street cleaning department relieved the health department and permitted the latter to apply its scanty funds where they would do more good.

The revelations of bacteriology and parasitology opened up a vast field of research and endeavor, which has been intensively and very profitably tilled ever since. In fact, in spite of all that we know about the importance of other causes of disease - nutritional, toxic, psychic - it still seems fortunate that the infectious causes should have been the first to have been intensively studied by scientific methods. Most diseases present vicious circles, or a series of vicious circles. Infection leads to disease, disease to poverty and ignorance, possibly to crime; poverty and ignorance predispose to malnutrition, toxaemias, and psychic depression; and these, in turn, predispose to infection. The circle could be represented in various ways and could be amplified ad infinitum, but this illustrates what is meant. The most vulnerable point in this circle still appears to be the infection link. Its elimination has not been accomplished, but a very decided impression has been made in the short space of roughly half a century. A general death rate cut in half indicates that a powerful influence has been at work.

As has been hinted, the conception of the proper scope of health work has undergone a progressive broadening. Without relinquishing research or control measures in sanitation and communicable diseases, it has passed on to the consideration of other important aspects. The food of the masses has become recognized as a determining factor of no small importance to their health. The discovery of the vitamins gave as much impetus to activity along this line as the recognition of the role of bacteria did to that concerned with infection. For, although there might have been some degree of actual starvation among a submerged portion of the population, it was not the deficiency in calories which caused a rather widespread state of malnutrition, especially among the lower age groups. Again, with the development of new industrial processes and the distribution all over the country of new articles and preparations, it was realized that a real health menace existed in the possible poisoning, both among the industrial groups and the general public, of the makers and users of some of these unfamiliar wares. Growing interest has also been manifested in mental disease as a cause of suffering and incapacitation, and the realization is at hand that something may and should be done to prevent or control this condition. Recently cancer has enlisted the consideration of public health officials as a condition deserving of, and to a degree amenable to, public health activities.

The student of public health is continually impressed with the immense significance of social and economic conditions to health, and health workers are increasingly employing an approach along lines suggested by this consideration.

With the foregoing summary by way of orientation, it is now necessary, in portraying the outlook of public health work, to examine in some detail various factors to which a brief allusion has been made.

Official public health work throughout the world is definitely committed to the acceptance and employment of the findings of science. This is not to say that there already exists a good scientific reason for every activity which is now carried out. There are still many gaps in knowledge which leave to the public health official the choice between doing nothing at all or doing what seems to him the most sensible thing, although he may not be able to quote solid scientific reasons for it. As soon as it becomes evident that these methods should be abandoned or replaced, or that they can be refined and improved, he is not, as a rule, slow to take appropriate action. For it must be remembered that as a public official he is always under the spurs of public opinion and contemporary criticism.

It is difficult to think of a branch of physical science which, if it does not have a bearing upon public health work to-day, may not have one to-morrow. Few would have thought, fifty years ago, that the work of an entomologist, patiently classifying mosquitoes as a labor of pure science, of an obscure monk observing the results of cross-fertilization of garden flowers, or of many a worker in chemical laboratories, engaged in replacing atoms in molecular chains, would now have commonplace applications in the daily work of health officials everywhere. No one can foresee what new and useful applications await future adoption.

Public health deals primarily with man as Homo sapiens. The diversity of belief and viewpoint which at least the American community presents from the religious, philosophical, traditional, and ethical standpoints makes it virtually impossible for the health official to regard his human population as anything more than a collection of more or less intelligent animals. If he were inclined to inject into practice any purely moral considerations of his own, he would soon be attacked from all sides. For example, he had better regard the moral and religious issues of birth control and of venereal disease as none of his business - that is, none of his public business - and stick to what scientific facts are available when it comes to practical application.

Now Homo sapiens is a biological entity, and it follows naturally that the science of greatest import to public health work is biology. Taking everything into consideration, it is probably true that biology has thus far contributed more to human welfare through public health work than any other science has. We are aware that chemistry has made great claims for recognition in this direction, and that we could not go far in our daily work without its contributions. We confidently believe, moreover, that chemistry has only begun its beneficent revelations; but up to date it is believed that the foregoing statement is true.