This paper examines the concept of “One Health,” which was established in 2000 by international organizations to prevent the global spread of infectious diseases due to disruptions in the human-animal-environment interface. Although One Health regards the health of humans, animals, and the environment as inextricably linked, there are in fact many causal factors that contribute to the spread of infectious diseases like COVID-19. This article identifies one causal factor with the “closure” by system differentiation of modern society. All sectors of society have become specialized, and perform their unique functions with exclusivity. As all sectors have their own expertise, irreplaceability has been normalized, and multidisciplinary and multi-organizational cooperation has become difficult. One Health is possible only if it transcends this system closure. However, despite national and international efforts, cooperation remains difficult to achieve because of the closed systems of communication within these different government agencies and sectors of the economy. What is needed is recognition of the fact that the human-animal-environment interface is a single unit and, therefore, that individual organizations, government agencies, or even fields of research cannot solve these complex problems on their own.