This chapter examines what impact the Basel Mission doctors had on scientific debates in the field of tropical medicine. Their studies served as trials from the periphery in a period when the discipline was undergoing processes of consolidation and professionalisation in European metropoles. Tropical medicine practised abroad, however, remained an amalgam combining germ theory with environmental disease theories and laboratory medicine with field surveys. In West Africa, the theory of human germ carriers culminated in the segregation of whole cities around 1900. The Basel medical missionaries advocated for the segregation of European and African housing areas, which was met with fierce resistance by their fellow missionaries. Their conflicting views on whether preventative measures to protect European lives should take precedence over the proclamation of the gospel in physical proximity to Africans indicate that scientific guidelines of hygiene, colonial notions of cleanliness and religious purity ideals also presented fundamental tensions that proved difficult to reconcile.