In the early 1900s, Washington, D.C. contained many alleys in the interior of blocks inhabited by impoverished Black residents. Elite reformers engaged in an aggressive campaign to eliminate alleys, on the grounds of their purported unsanitary environment and high disease prevalence. In this paper, I combine quantitative, qualitative, and spatial sources to explore new perspectives on segregation, public health, and the racialized efforts of housing reformers during this period. I find that reformers overstated the horrors of conditions in alleys and their effects on residents’ health: poorer health among alley residents was in large part due to Black residents’ marginalization wherever they might live. Alleys’ status as racialized space, coupled with progressive paternalistic racism, facilitated the discursive construction of alleys as pathological “breeding grounds of disease.” Further, my findings shed new light on micro-configurations of segregation within racially mixed neighborhoods, as well as the social experience and meaning of such configurations. Far from indicating harmonious coexistence, the proximity of such alleys to white homes and institutions spurred elite Washingtonians’ self-interested fear of disease spreading beyond the alleys. Thus, this pattern of segregation helps explain the zeal of the campaign to eradicate alleys: as a means of achieving separation from undesired Black neighbors whom white reformers associated with contagion.