PUBLIC HEALTH, or the health of the people, i.e., of groups or communities of human beings, is to-day a great and growing subject which may be regarded either as a science— and is then nearly or quite synonymous with public hygiene — or as a state or condition of the physical well-being of human groups or societies. As a science, it has of late years, and especially since the great pathological dis coveries of the last quarter of the 19th century, taken on new forms and extraordinary import ance. As a state or condition, it has also re cently attracted much attention in the improve ments shown in almost all civilized countries. As a science, it is beginning to be recognized in schools and universities by the establishment of new courses, special teachers and even special training schools — notably at Harvard University and the Massachusetts Institute of Technology, at Yale and at the Tohns Hopkins University — an early step in this direction hav ing been taken at the University of Pennsyl vania. As a condition, worthy of and demand ing world-wide attention, it has led to the estab lishment of a Federal Bureau in the Treasury Department of the United States government (formerly the United States Marine Hospital Service, later the United States Marine Hos pital and Public Health Service and now the United States Public Health Service, having at its head a surgeon-general with several assistant surgeons-general, with a large and important hygienic laboratory (in Washington) and a strong staff of medical men, sanitary chemists, sanitary biologists and sanitary engineers. Private and voluntary associations in great numbers, such as the International Health Board of the Rockefeller Foundation, the American Public Health Association, the American Association for the Study and Prevention of Tuberculosis, the American Society for the Prevention of Infant Mortality, and many others far too numerous to mention, have also been formed and are generally ac tively at work to promote and improve the pub lic health. The latest step of great magnitude and promise in this direction is the diversion of some of the vast machinery and effort of the American Red Cross remaining after the World War into channels of public health education and sanitary and hygienic betterment. It is plain that the problems of individual or per sonal health or hygiene are sufficiently complex and uncertain, both of determination and amelioration, but if, instead of dealing with the individual, we deal with groups or masses of individuals in the form of communities, such as families, villages, towns, cities, States and nations, all the difficulties (and these arc by no means few) connected with individual hygiene become enormously multiplied and complicated.
It is easy to see that when the problems of the individual under varying conditions dealing with a variable environment are multiplied and com plicated by the introduction of those of other individuals, with the inevitable effects which these produce upon the common environment, the public health problems with which we have to deal are by no means either simple or easy of solution. We need in the first place a precise knowledge of the quantity of life (that is, the population) with which we have to deal, and if over against this we set the total amount of dying within a particular period and then deter mine the death-ratio (or death rate), we shall have at least a crude and general measure of the sanitary condition of the community. But if we determine simply the general death rate, we shall not have gone far enough, for it is now well understood that the death rate is pro foundly influenced by factors other than dis ease, such for example as the birth rate, a high birth rate naturally leading under present condi tions to a high death rate. It becomes neces sary then in any scientific examination of pub lic health problems to go behind the general death rate and to study other conditions. One of the most important of these is the so-called specific death rate, that is to say, the death rate in any given population from particular dis eases such as tuberculosis, typhoid fever, pneu monia and the like; for it not infrequently hap pens that in the presence of a fair general death rate the mortality from typhoid fever (for ex ample) may be excessive. In determining popu lation the census is of the first importance (see CENSUS) and a well-directed census should furnish not merely the total number of the liv ing but the gage periods> of the population, in asmuch as the mortality at different ages varies widely, and no safe conclusions concerning the sanitary conditions of a population can be drawn without careful reference to the constituents of that population at the different age-periods. Space will not permit here a detailed treatment of the sources of error to which students of salutation, morbidity, death rates and the public health are exposed in various other directions, but which reference to any of the leading works upon vital statistics will readily disclose.