The organized effort of societies to protect and improve the health of populations as a whole, rather than treating individuals one at a time. Public health rests on the insight that the conditions in which people live — water, air, housing, food, income — shape health outcomes more powerfully than medicine alone.
In 1854, a physician named John Snow removed a pump handle in a London neighborhood and stopped a cholera epidemic. He had no germ theory. He had no treatment. He had a map showing where the sick lived, a hypothesis about contaminated water, and the authority to act on it. That intervention — not a cure, but a structural change to a shared environment — is the founding act of modern public health. It works by changing the conditions that make people sick before they get sick at all.