Public health consists of the activities of a society that protect and advance the health of its people. Public health resources consist of a set of scientific disciplines, practical skills, and beliefs (values) that are directed to the maintenance and improvement of people's health through collective or social action; as well as the political will that is necessary to mobilize resources and people in support of the agencies that carry out the activities. Typically, though not in all nations, public health operates at local, state, national, and international levels—with coordination, collaboration, and interaction among these four levels. It takes many forms, including governmental, nongovernmental (voluntary), and professional.

Public health work is conducted by teams that include physicians, nurses, dentists, epidemiologists, social workers, behavioral scientists, health inspectors, sanitary engineers, statisticians, administrators, supporting clerical and secretarial staff, and others. Efficient and effective public health practice demands a high degree of harmonious collaboration among teams that vary greatly in size, complexity, levels of skill and expertise, financial support, and political commitment to the perceived importance of public health.

National leaders from Benjamin Disraeli and Otto von Bismarck to Franklin D. Roosevelt and John F. Kennedy have eloquently expressed the belief that public health is an important element, indeed is at the foundation, of national security. Protecting the health of a nation's people is as important as protecting against invasion by a foreign adversary. Yet in no nation do public health services command more that a tiny fraction of the total national expenditure on medical and health services—typically less than 5 percent of the health budget from all public and private sources is devoted to public health. In the world's poorest nations, where the public health problems are most oppressive, the proportion is seldom as much as 1 or 2 percent of all health expenditures. Many of the poorest nations spend far more of their slender resources on armaments—sometimes to be used against their own people—than on public health services.

In focusing on the health of populations, public health differs from the healing arts such as medical, nursing, and dental practice, which respond to the health needs of individuals. From earliest times society has thrown out two arms for health; one to protect its people (e.g., migration in search of better food supply), and the other to care for the sick. Beginning with the Industrial Revolution, when people flocked from the countryside into the cities for factory work, public health concentrated on combating the communicable diseases such as tuberculosis and enteric infections that flourished in the crowded, growing urban areas. Remarkable success has attended those efforts, though much remains to be done, especially in developing countries. With the decline of communicable diseases (despite occurrence of some new ones, such as HIV/AIDS) and the generation of a new lifestyle during the twentieth century, noncommunicable diseases such as heart disease and cancer have expanded—in the developing world as well as in industrialized countries. Other problems, such as drug addiction and domestic violence, are currently being socially redefined as public health issues.

In contrast to the major healing professions, public health personnel concentrate on the factors prevailing in a society that substantially affect the health of its people as a whole. Prominent among these are nutrition, environmental and occupational hazards, and lifestyle factors. Gross undernutrition is still a problem among segments of the population in developed nations and is an extensive problem in developing nations. Ironically, overnutrition, especially the consumption of animal fats, is increasingly and negatively affecting health throughout the globe. Water and air pollution, unsafe food, and workplace hazards pose dangers for most people in the world. Behaviors regarding tobacco, alcohol, physical exercise, and other daily habits—largely influenced by people's living milieu—are now recognized as major factors in the development of noncommunicable diseases.

Public health incorporates scientific advances into its armamentarium, demonstrating the role of microbic agents in communicable diseases; and the role of physical inactivity, eating excessive fats, and exposure to toxic chemicals as causes of noncommunicable diseases. Epidemiology—sometimes called public health's basic science—delineates health problems, contributes substantially to ascertaining their causes, and provides guidance toward solutions. Epidemiology and biostatistics serve as the "diagnostic" element of public health. Intervention or "therapeutic" segments consist of environmental, sociobehavioral, and personal health services, which focus on the physical and social milieu in which people live, as well as taking account of human biological nature.

After defining the mission of public health as "fulfilling society's interest in assuring conditions in which people can be healthy," the Institute of Medicine of the U.S. National Academy of Sciences outlined its three core functions: (1) assess-ment—the delineation of health problems, their nature, and the means of dealing with them; (2) policy development—the formulation and advocacy of what should be done about health problems; and (3) assurance—the implementation of policy, either by activities of others or by direct public health activities.

More specific roles of public health have evolved over time. For example, in the mid–twentieth century it was commonly accepted in public health circles that the "six basic functions" of public health were: vital statistics, communicable disease control, sanitation, laboratory services, maternal and child health services, and health education of the public.

In 1997 the World Health Organization (WHO) assembled a consensus document, based on discussions involving 145 prominent leaders of public health from sixty-seven nations, that specified nine categories of essential public functions:

1. Monitoring the health situation. This includes monitoring morbidity and mortality, the determinants of health (e.g., smoking), the effectiveness of public health programs and functions, and assessing population needs and risks.
2. Protecting the environment. Environmental protection includes ensuring access to safe water; the control of food safety and quality; provision of adequate drainage, sewerage, and solid waste disposal services; and the control of hazardous substances and wastes. Adequate vector control measures; protecting water and soil resources; and controlling atmospheric pollution and ionizing radiation are also essential, as is ensuring adequate preventive environmental services, and adequate inspection, monitoring, and control of environmental hazards.
3. Health promotion. The promotion of community involvement in health; the provision of information and education for health and life-skill enhancement in school, home, work, and community settings; and maintaining linkages with politicians, other sectors, and the community in support of health promotion and public health advocacy are all part of health promotion.
4. Prevention, surveillance, and control of communicable diseases. This function includes immunization, disease outbreak control, disease surveillance, and injury prevention.
5. Public health legislation and regulation. The review, formulation, and enactment of health legislation, regulations, and administrative procedures are also essential functions of public health. Components include ensuring adequate legislation to protect environmental health; health inspection and licensing; and enforcement of health legislation, regulations, and administrative procedures.
6. Occupational health. Setting occupational health and safety regulations, ensuring safety in workplaces, and providing medical and health services for workers are part of this function.
7. Specific public health services. These include school health services, emergency disaster services, and public health laboratory services.
8. Public health management. Management of public health involves ensuring health policy, planning, and management; the use of scientific evidence in formulating and implementing health policies; public health and health-systems research; and international collaboration and cooperation in health.
9. Care of vulnerable and high-risk populations. This involves maternal health care and family planning; infant and child care; and programs to protect the health of refugees, displaced persons, and aboriginal peoples.

Clearly some of these essential public health functions have higher priority than others. In a well-run state, nation, or community, all should be available. When they are not, or when they have previously existed and have fallen into disrepair— as has happened in the former Soviet Union— epidemic diseases such as diphtheria and poliomyelitis soon return, endangering not only the local people but people everywhere. In addition, as noted previously, noncommunicable diseases now require attention in the developing nations, where it is estimated that their mortality currently exceeds that of the communicable diseases, and in the developed nations, where neoplasms and cardiovascular diseases cause about two-thirds of all deaths. Moreover the fact that many of the essential functions require the participation of other sectors of society helps to reinforce the notion that public health not only calls for teamwork and collaboration, it also is everybody's business.

LESTER BRESLOW
BERNARD D. GOLDSTEIN
LAWRENCE W. GREEN
C. WILLIAM KECK
JOHN M. LAST
MICHAEL MCGINNIS