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Yellow fever, a mosquito-borne flavivirus, was inescapable in the 19th-century Deep South and a point of near-constant terror in New Orleans, the region’s hub. In the six decades between the Louisiana Purchase and the Civil War, New Orleans experienced 22 full-blown epidemics, cumulatively killing over 150,000 people. (Perhaps another 150,000 died in nearby American cities.) The virus killed about half of all those it infected and it killed them horribly, with many victims vomiting thick black blood, the consistency and color of coffee grounds. The lucky survivors became “acclimated,” or immune for life.

Antebellum New Orleans was a slave society where whites dominated free people of color and enslaved people through legally sanctioned violence. But another invisible hierarchy came to co-mingle with the racial order; white “acclimated citizens” stood atop the social pyramid, followed by white “unacclimated strangers,” followed by everyone else. Surviving yellow fever was locally known as the “baptism of citizenship:” proof that a white person had been chosen by God and had established himself as a legitimate and permanent player in the Cotton Kingdom.

Immunity mattered. “Unacclimated” white people were considered unemployable. As the German immigrant Gustav Dresel lamented in the 1830s, “I looked around in vain for a position as bookkeeper,” but “to engage a young man who was not acclimated would be a bad speculation.” Life insurers rejected unacclimated applicants outright or else charged a hefty “climate premium.” If you were white, immunity-status impacted where you lived, how much you earned, your ability to get credit, and whom you were able to marry. It’s no wonder, then, that many new immigrants actively sought sickness: huddling together in cramped dwellings, or jumping into a bed where friends had just died — the antebellum forerunners to “chickenpox parties,” except much deadlier.

But immunity was more than a product of epidemiological luck. In the context of the Deep South, it was wielded as a weapon. From the start, wealthy white New Orleanians made sure that while mosquitoes were equal-opportunity vectors, yellow fever would be anything but colorblind. Pro-slavery theorists used yellow fever to argue that racial slavery was natural, even humanitarian, because it allowed whites to socially distance themselves; they could stay at home, in relative safety, if black people were forced to labor and trade on their behalf. In 1853, the “Weekly Delta” newspaper claimed, ludicrously, that three-quarters of all deaths from yellow fever were among abolitionists.

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