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Trump is hardly the first to use the words he chose to describe whole populations: Ideas about infestation have long been weaponized to justify the systemic discrimination of African Americans. The term carries a long history of financial gain through racist dispossession. And that phenomenon has a special resonance in Baltimore.

Over the 20th century, Baltimore became a laboratory for white policymakers to experiment with implementing racial discrimination. Often, the process was justified with the coded language of rooting out infestation. On the surface, this might have looked like race-neutral language that was addressing disease, rodents, or environmental hazards. But scratch that surface and it quickly becomes clear that the city’s leaders were using the language of public health to restrict where African Americans could live.

As I write in my forthcoming book Building Suburban Power: The Business of Exclusionary Housing Markets, one of the most infamous and widely used tools of residential racial segregation had its origins in Baltimore. At the turn of the 20th century, suburban development companies bought large tracts of land on the periphery of the growing city. Among the most influential was the Roland Park Company, which created some of Baltimore’s first restricted communities governed by a new tool: the restrictive covenant. These documents were legal contracts that had historically been applied to single lots. The Roland Park Company had a different idea: make them community-wide.

To do that, they looked to existing “nuisance laws” that cities used to regulate property in order to prevent health hazards such as rat infestations. The Roland Park Company filled its covenants with rules about what property owners could and could not do. In the middle of the covenants’ nuisance clause, the company added a racial restriction—the first of its kind in Baltimore. The restriction, which prohibited blacks from living in Roland Park subdivisions, was sandwiched between environmental regulations about livestock and smoke. The company widely disseminated the restrictive covenants throughout the country. Racially restrictive covenants, as a result, are derived from language that directly pertained to animal infestation.