When the constitution of the World Health Organization entered into force after ratification by 26 member states, it created the United Nations' dedicated health agency, headquartered in Geneva. The date is now marked annually as World Health Day. Over subsequent decades the WHO would coordinate the eradication of smallpox, battle polio, and become the central global authority during pandemics from HIV/AIDS to COVID-19.
The WHO's founders, meeting in the aftermath of a war that had exposed how disease, famine, and displacement moved as freely across borders as armies, wrote an unusually ambitious preamble: health as 'a state of complete physical, mental and social well-being,' not merely the absence of disease. That expansive definition still shapes — and still provokes debate about — the organization's mandate today.
Its first great triumph came with smallpox. Beginning an intensive vaccination and surveillance campaign in the 1960s, WHO teams tracked outbreaks village by village across Africa and Asia; the last natural case was recorded in Somalia in 1977, and in 1980 the organization declared the disease eradicated — the only human disease ever eliminated entirely by human effort.
The agency's reputation has waxed and waned since: praised for near-eliminating polio and Guinea worm disease, criticized for slow responses to Ebola outbreaks and for early missteps during COVID-19. Each April 7, World Health Day gives the organization a platform to spotlight a chosen theme — mental health, universal coverage, climate and health — reasserting the founders' conviction that disease respects no border and requires collective response.