When the Europeans settled in the Carolinas, they brought malaria with them. The ensuing epidemic affected both the natives and the settlers (and malaria, with

When the Europeans settled in the Carolinas, they brought malaria with them. The ensuing epidemic affected both the natives and the settlers (and malaria, with lethargy as its major symptom, is especially bad for a manual labor force). People of West African descent tend to have a very strong resistance to malaria that's related (in a way I don't understand) to the sickle-cell gene. Since the ships trading back and forth across the Atlantic were also trading to the western coast of Africa (and exchanging sailors/slaves/passengers at all stops), eventually some west African laborers ended up in the carolinas, and someone noted that they were unaffected by malaria. The eventual presence of large groups of west african laborers, combined with a tradition/economy of chattel slavery among the Carolina natives that predated the settlers to create a very economically effective slavery system that spread across the southern US.