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By Root 3161 0

cells, falciparum, unlike vivax, manages to alter them so that they stick to the walls of the tiny capillaries inside the kidneys, lungs, brain, and other organs. This hides the infected cells from the immune system but slowly cuts off circulation as the cells build up on the capillary walls like layers of paint on an old building. Untreated, the circulation stoppage leads to organ failure, which kills as many as one out of ten falciparum sufferers. Vivax doesn’t destroy organs, and thus is less deadly. But during its attacks sufferers are weak, stuporous, and anemic: ready prey for other diseases. With both species, sufferers are infectious while sick—mosquitoes that bite them can acquire the parasite—and can be sick for months.

dium, a tropical beast, is exquisitely sensitive to temperature. The speed at which the parasite reproduces and develops in the mosquito depends on the temperature of the mosquito, which in turn depends on the temperature outside (unlike mammals, insects cannot control their own internal temperature). As the days get colder, the parasite needs more and more time to develop, until it takes longer than the mosquito’s lifespan. Falciparum, the most deadly variety of malaria, is also the most temperature sensitive. Around 72°F it hits a threshold; the parasite needs three weeks at this temperature to reproduce, which approaches the life expectancy of its mosquito host; below about 66°F it effectively cannot survive. Vivax, less fussy, has a threshold of about 59°F.

Unsurprisingly, falciparum thrives in most of Africa but gained a foothold only in the warmest precincts of Europe: Greece, Italy, southern Spain, and Portugal. Vivax, by contrast, became endemic in much of Europe, including cooler places like the Netherlands, lower Scandinavia, and England. From the American point of view, falciparum came from Africa, and was spread by Africans, whereas vivax came from Europe, and was spread by Europeans—a difference with historic consequences.

Human malaria is transmitted solely by the Anopheles mosquito genus. In Jeake’s part of England the principal “vector,” as the transmitting organism is known, is a clutch of tightly related mosquito species known jointly as Anopheles maculipennis. The mosquito’s habitat centers on the coastal wetlands of the east and southeast: Lincolnshire, Norfolk, Suffolk, Essex, Kent, and Sussex counties. A. maculipennis—and the dium vivax it carries—seem to have been uncommon in England until the late sixteenth century, when Queen Elizabeth I began encouraging landlords to drain fens, marshes, and moors to create farmland. Much of this low, foggy terrain had been flooded regularly by the North Sea tides, which washed away mosquito larvae. Draining blocked the sea but left the land dotted with pockets of brackish water—perfect habitat for A. maculipennis. Farmers moved into the former marsh, still soggy but now usable. Their homes and barns, heated during cold weather, provided space for the mosquito—and the vivax parasites inside its body—to survive the cold weather, ready to breed and spread in the following spring.

As the British medical historian Mary Dobson has documented, draining the marshes set off an inferno of vivax malaria. Visitors to maculipennis habitat recoiled at the wretchedness they encountered. An all-too-typical sight, lamented the Kent writer Edward Hasted in 1798, was “a poor man, his wife, and whole family of five or six children, hovering over the fire in their hovel, shaking with an ague [fever], all at the same time.” Curates died in such numbers after being sent to coastal Essex, the writer John Aubrey remarked, that the area was known as “Killpriest.” Natives fared no better; babies born in the marshland, Hasted wrote, seldom “lived to the age of twenty-one.” Dobson recorded baptisms and burials in twenty-four wetland parishes. In the 1570s, before Queen Elizabeth drained the swamps, baptisms exceeded burials by 20 percent—the population was rising. Two decades later draining was in full swing, and burials outnumbered baptisms by almost a factor