'A Biological Side Effect': Yaws Eradication, Syphilis, and Debates over 'Cross-Immunity' at the World Health Organization, 1948–1990.

This article examines the World Health Organization’s (WHO) mid-twentieth-century global campaign to eradicate yaws, a non-venereal treponemal disease, and its unintended epidemiological consequence: the rise of syphilis, a related but sexually transmitted infection. While the WHO’s yaws eradication efforts (1952–1964) successfully reduced yaws prevalence by 95% across 46 countries, accumulating evidence from laboratory studies and field reports indicated that the loss of yaws-induced cross-immunity increased populations’ susceptibility to syphilis. Despite acknowledging this risk, the WHO gradually downplayed the cross-immunity hypothesis by the 1970s, attributing syphilis’s spread instead to socio-cultural changes such as urbanization and the “sexual revolution,” often framed through racialized and moralistic discourses. The article situates these developments within broader patterns of post-war global health initiatives, highlighting institutional reluctance to address adverse side effects of eradication campaigns and the persistence of colonial-era attitudes in disease control narratives.