The paper focuses on the role of the World Health Organization (WHO) in promoting a healthy world population as a generative and robust idea within health policy. The WHO’s health credo transcends national boundaries to promote health globally. It is embedded in norms, values, and standards promulgated by the organization and contributes in shaping the health responses of national governments. Ideational robustness refers to the ability of the WHO to adapt its health credo to changing contexts and circumstances, thus promoting the legitimacy of an international health order. Disturbances, including the Covid-19 pandemic, test the credo’s robustness, forcing the WHO to constantly work at reframing ideas to adapt to political forces and competing logics that structure the field of international health. Empirically, the paper is based on an historical analysis of the evolution of the health credo of the WHO since its inception. Qualitative content analysis of secondary sources, such as policy documents, explores how ideational work performed by WHO leaders impacts on the organization’s position and legitimacy. Ideational robustness appears to be largely influenced by leadership vision, preexisting organizational structure, and the political economy of international health. Ideational robustness appears as a powerful yet insufficient ingredient of policy success.