A new disease called Zika fever ravaged Brazil in 2015, and is beginning to spread north to the Caribbean, Mexico, and the Gulf Coast of the United States. Zika is transmitted through the bites of the Aedes aegypti mosquito, a creature responsible for spreading diseases in the Americas for centuries. The mosquito and the new disease it transmits pose a dire public health concern for the people of the Americas. Zika is also part of the legacy of A. aegypti-borne disease and inequality, rooted in slavery and most dramatically exemplified during the reign of yellow fever.

Researchers first discovered the Zika virus in an infected monkey from the Zika forest in Uganda in 1947. The virus soon spread to human populations in Africa, Indonesia, and the Pacific islands. The first reported cases of Zika in the Americas appeared in Brazil last year, likely as a result of infected individuals traveling to the World Cup. Scientists have linked the virus to microcephaly, a condition that renders newborns with abnormally small heads. Thousands have been affected and more than forty have died already, while the rest face lifelong cognitive impairments. Like other mosquito-borne diseases, its chief victims are in poverty-stricken areas where the lack of indoor plumbing, and drought brought on by climate change, force the poor to keep water in rain barrels and other artificial containers where A. aegypti breed. The disease has Brazilians in panic and public health officials urging women to delay pregnancy.

Long before Zika arrived on western shores, A. aegypti had settled in the sultry environments of the Caribbean, Central, and South America. The mosquitoes made the transatlantic passage as stowaways, breeding in the containers of water that slave ships carried for their unfortunate passengers. Once in the New World, A. aegypti flourished near Caribbean and South American sugar plantations. The first unmistakable evidence of its presence came in 1647 when yellow fever struck on Barbados and the Yucatan Peninsula. Yellow fever is a disease that induces internal hemorrhage and kills between ten and fifty percent of victims. The virus that causes it also came from Africa and arrived in the New World via the slave trade, embedded in the bloodstreams of slaves and slave-drivers.

The mosquito and yellow fever virus formed a deadly pair. The habits of A. aegypti and the dynamics of human settlement in the New World resulted in intermittent cycles of yellow fever epidemics. Since the mosquito breeds in artificial containers, it lingered near human habitations. Whenever the population of non-immune hosts grew high enough, mosquitoes spread the virus from host-to-host. Often the available pool of susceptible victims either died or gained immunity to the disease, causing the virus to disappear. In other cases, the disease shifted into a jungle phase, where it survived in monkey populations. The long-term destructiveness of yellow fever required the frequent reintroduction of the yellow fever virus for it to survive, a need easily sustained by the traffic of slaves between Africa and the slave markets of the New World.

Yellow fever went wherever slavery went, and often as far as the goods slaves produced. From the seventeenth century to the end of the eighteenth century, the disease terrorized the Caribbean islands and Brazil, where brutal conditions of the sugar mills required the constant infusion of new laborers. Despite their greater resistance to tropical diseases, African laborers made up clear majorities in sugar producing regions, and so they suffered most from them. The disease also extended beyond the habitable zone of A. aegypti, striking as far north as Philadelphia and New York, major destinations of goods from the Caribbean. Residents of these cities paid a dearer price for their sugar than they imagined.

Yellow fever epidemics in the northeastern United States provoked vigorous public health responses. Well-funded health authorities imposed quarantine regulations that effectively policed ship traffic and instituted sanitary reform measures that unwittingly destroyed breeding grounds for A. aegypti. By the early nineteenth century, the mosquito lost its tentative connection to the cool North, never a place it could survive the winters. Throughout the nineteenth century, yellow fever still tormented the more humid South, where a budding sugar culture in places such as Louisiana created ideal circumstances for A. aegypti. Yellow fever’s reign in the United States all but ended when researchers in the early 1900s identified the vector for yellow fever and set in motion mosquito eradication measures that limited A. aegypti to a thin band along the Gulf Coast.

Not everyone was so lucky. Today, A. aegypti still lurks in the tropical environs of the New World and the Old. Underdeveloped infrastructures and ineffectual, corrupt governments mean that it will likely stay. Yellow fever seldom erupts into wide-wasting epidemics, but it continues to imperil millions along with dengue fever, chikungunya, and now Zika fever. The poor and the marginalized still bear most of their burden. Zika is a problem for this age, but it is also a legacy of the slave system that molded the environmental and social landscapes of the Americas, and a sad reminder of the stark inequalities that divide our world.