Between 1545 and 1548, an epidemic swept through the indigenous people of Mexico that is unlike anything else described in the medical literature. People bled from their face while suffering high fevers, black tongue, vertigo, and severe abdominal pain. Large nodules sometimes appeared behind their ears, which then spread to cover the rest of their face. After several days of hemorrhage, most who had been infected died.
The disease was named cocoliztli, after the Nahautl word for “pest.” By contemporary population estimates, cocoliztli killed 15 million people in the 1540s alone—about 80 percent of the local population. On a demographic basis, it was worse than either the Black Death or the Plague of Justinian. For several centuries, its origin remained a mystery.
Then, about two decades ago, researchers began to compare the known cocoliztli outbreaks with clues etched in the tree rings of modern-day Mexico.