public health, noun, the science and practice of protecting and improving the health of a community,
as by preventive medicine, health education, control of communicable diseases,
application of sanitary measures, and monitoring of environmental hazards.

----   American Heritage Dictionary

The New World Encylopedia calls public health a “field of medicine and hygiene dealing with the prevention of disease and the promotion of health by government agencies.”

Another definition of public health is “the promotion of health and prevention of disease through the organized efforts of society."

The U.S. Institutes of Medicine says, "Public health is what we, as a society, do collectively to assure the conditions for people to be healthy. This requires that continuing and emerging threats to the health of the public be successfully countered. These threats include immediate crises, such as the AIDS epidemic; enduring problems, such as injuries and chronic illness; and growing challenges, such as the aging of our population and the toxic by-products of a modern economy, transmitted through air, water, soil, or food. These and many other problems raise in common the need to protect the nation's health through effective, organized, and sustained efforts led by the public sector."

But no matter how you define public health, the former U.S. Surgeon General C. Everett Koop summed up the importance of public health: “Health care is vital to all of us some of the time, but public health is vital to all of us all of the time.”