Spillover: the powerful, prescient book that predicted the Covid-19 coronavirus pandemic.

by David Quammen

Ciuca evidently felt that Plasmodium knowlesi might be a better weapon against neurosyphilis than other kinds of the parasite. He inoculated several hundred patients and, in 1937, reported fairly good success. His program of treatments continued until, almost twenty years later, a problem arose. Repeatedly passaging P. knowlesi through a series of human hosts (injecting infected blood, allowing the merozoites to multiply, and then extracting infected blood) had made Ciuca’s strain increasingly virulent—too virulent for comfort. After 170 such passages, he and his colleagues became concerned with its growing ferocity and stopped using it. That was a first cautionary signal, but still just a laboratory effect. (Passaging was necessary for replenishing a supply of the parasite, since it couldn’t be cultured in a dish or a tube; but passaging it directly through humans liberated the parasite from whatever different evolutionary pressures had been entailed in completing its life cycle within mosquitoes. It became like the protist equivalent of a designated hitter—very capable of batting, and freed from the responsibility to play outfield.) Other evidence would eventually show that P. knowlesi could be dangerous enough to humans in its wild form.

Poison turned medicine turned poison again