Taken in a broad sense, biology includes a large number of branches. The space limits of this article permit reference to only some of the more striking applications of biological sciences in public health.

First of all, as regards man himself it is self-evident that all we can learn regarding his anatomy, physiology, and psychology is bound to have applications in a program which concerns itself with his health. The first two have been upon a solid scientific basis for many years and are being built up as fast as, or faster than, practical application can be made of their findings. Psychology, on the other hand, can hardly be described as more than emerging from the realms of a purely speculative discipline, although with the increase of interest which is everywhere manifest and the tendency to supplant introspection with experiment and comparative considerations, the outlook is most hopeful. Public Health has much need of psychology. These sciences which concern themselves with the so-called normal serve as the point of departure for the study of abnormal or diseased conditions in which we have the subjects of pathological anatomy, -physiology, and -psychology, and their numerous specialties.

Still dealing primarily with man, and of immense importance to public health, are certain considerations of sociology. In fact, it is approximately true that if sociology were able to solve some of the problems of man's communal existence, public health would find its problems reduced by half. For example, if the economic status of a portion of the population could be improved by 100 per cent, which it certainly needs to be, public health could guarantee the virtual elimination from it of such diseases as malaria and pellagra within a year or two.

The science of epidemiology, properly so designated within the past few decades, deals also primarily with man, although secondarily it draws in a great number of related subjects. It is a branch of demography - the study of disease in populations. Epidemiology, which in its methods deals first with the individual, and then assembles and analyzes similar data concerning a large number of individuals, has proved fruitful in recent years not only in permitting the formulation of generalizations concerning diseases, but also in pointing to the most profitable line of attack, both in research by laboratory methods and in the application of control measures. For example, epidemiology shows pellagra to be a disease essentially of the poor, not transmissible, seasonally limited, and associated with dietaries of scanty variety. Laboratory studies directed along the lines so suggested prove that the disease is caused by the deficiency of a particular food principle in the diet, and that it may be prevented and cured by providing foods containing that principle. As another example, poliomyelitis (infantile paralysis) may be cited. Epidemiology shows that it spreads in populations after the manner of infections, disregards social and economic status, and is characterized during epidemics by the production of an enormously disproportionate number of light cases and healthy carriers. On the basis of this information public health must recognize the futility of attempts to prevent its spread by restraining the liberties of individuals, and must concentrate on the early recognition of cases and the institution of appropriate treatment, and on research to improve or perfect present methods of specific prevention and treatment.

An enormous proportion of human disease is due to the fact that we live in a world containing other forms of life with which we are in constant competition. These forms are both animal and vegetable, and they affect health in a great variety of ways. Poisonous snakes and plants, for example, have in some times and places been serious health menaces, and a great deal of study has been necessary in order to develop protective measures. With the advance of civilization, however, the emphasis has changed from these external and obvious dangers to the more insidious and difficult problems of parasitism. Parasites are animals or plants, readily visible or microscopic in size, which live in the body at its expense. In many cases they exert little harmful effect beyond diverting a portion of the food supply from the tissues. In others they produce poisons which irritate, disarrange, or even destroy the tissues of the host. The bacteria, for example, which are microscopic plants, are capable, according to their species, of producing such specific diseases as cholera, plague, typhoid fever, dysentery, pneumonia, and a host of others.

There is hardly any fact which can be ascertained by scientific means concerning the myriad forms of parasitic plants and animals which may not find some application in medicine and public health. Physicians and health officials, recognizing the importance of such knowledge to their specialties, have therefore supplemented the studies of zoologists and botanists by researches of their own, and in many branches have well repaid, by their contributions, the great debt which they owed to the biological sciences. For example, they have virtually created the subject of medical bacteriology, have made important contributions to entomology, and may safely claim as their own the sciences of serology and immunology.

The dependence of medicine and public health upon the biological sciences is so great that it is futile to attempt a list of even the special subjects concerned. The case may be briefed by the statement that without the sciences of zoology and botany there would be no public health.

The contributions of chemistry also are innumerable, and they have recently been ably and amply presented to the public in many attractive forms. It is sufficient to allude to the indispensable part played by chemistry in physiology in leading to an understanding of bodily functions, both normal and diseased; to the applications of chemical dyes in the study of bacterial and other parasites; and to the preparation of valuable drugs which play a great part in health work.