This article shows how French doctors based in Saint-Louis-du-Sénégal, the capital of colonial Senegal, conceptualised the Senegambian region as a diseased environment and Africans as carriers of infectious agents. It explains how perceptions of the hot tropical climate, combined with outbreaks of epidemic diseases and seasonal allergies, were instrumental in the processes of urban transformation through hygienic measures such as waste removal, the closing of cemeteries, and the imposition of new building codes. The article also shows how the stigmatisation of Africans was implicated in the forced removal of the urban poor – firstly from the city centre, and later from the entire city-island. Colonial medical knowledge in Senegal was initially based on the miasma theory, however, germ theory was adopted in the aftermath of the 1900 yellow fever epidemic. Both theories, in relation with racialism, impacted the urban landscape in Saint-Louis, Senegal.