The 1948 constitution of the World Health Organization (WHO) defines health as ‘a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity’. It was a bold and revolutionary health idea to gain international consensus in a period characterized by fervent anti-communism. This article explores the genealogy of the health definition and demonstrates how it was possible to expand the scope of health, redefine it as ‘well-being’, and overcome ideological resistance to progressive and (...) international health approaches. The first part of the article demonstrates how the health definition was composed through a trajectory of draft ideas from scholars in the history of medicine, as well as political actors working to promote national health insurance. The definition was authored by League of Nations veteran Raymond Gautier, but secretly drew heavily on medical historian Henry E. Sigerist’s controversial book Socialized Medicine in the Soviet Union (1937). The second part analyses how it was possible to resist the ideological pushback against the WHO and secure US ratification. The WHO’s progressive constitution was not simply a deviation from dominant health ideas, but a direct outcome of the entrenched health conflict. The genealogy is based on original archival material from international organizations and US government archives. The article contributes to understandings of the political controversies surrounding the WHO and to scholarship on understandings of health. It also illustrates how influential health ideas cross the boundaries between politics and health sciences, as well as the boundaries between domestic health policy and global health. (shrink)