Antebellum New Orleans sat at the heart of America’s slave and cotton kingdoms. But it was also the nation’s “necropolis,” with yellow fever routinely killing about 8 percent of its population. With little epidemiological understanding of mosquito-borne viruses—and meager public health infrastructure—a person’s only protection against the scourge was to “get acclimated”: fall sick with, and survive, yellow fever. About half of all people died in the acclimating process. Repeated epidemics generated a hierarchy of immunocapital whereby “acclimated citizens” (survivors) leveraged their immunity for social, economic, and political power and “unacclimated strangers” (poor recent immigrants) languished in social and professional purgatory. For whites, acclimation was the quintessential demonstration of calculated risk-taking: that people had paid their biological dues, were worthy of investment, and could now justifiably pursue economic advancement in slave racial capitalism. For black slaves, who were embodied capital, immunity enhanced the value and safety of that capital for their white owners, strengthening the set of racialized assumptions about the black body bolstering racial slavery. By fusing health with capitalism, this article presents a new model—beyond the toxic fusion of white supremacy with the flows of global capitalism—for how power operated in nineteenth-century Atlantic society.