Scribal Terror: Cocolitzli

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The epidemics of 1545 and 1576 killed millions of Aztecs and forever changed the demography of Mexico. Until now, most historians assumed that the disease responsible for these deaths was either smallpox or typhus brought to the New World by Cortés. An epidemiologist named Rodolfo Acuña-Soto, however, believes that the "megadeath" was caused by a native hemorrhagic fever called cocolitzli. Bruce Stutz of Discover magazine has written a fascinating article called "Megadeath in Mexico" which summarizes Acuña-Soto's findings. Interestingly, much of Acuña-Soto's research is based on tree-ring data:

The evidence from the Douglas firs shows that during the 16th century central Mexico not only lacked rain but also suffered the most severe and sustained drought in 500 years, one that encompassed nearly the entire continent. Moreover—here was Acuña-Soto's smoking gun—the tree-ring records show wet interludes setting in around the years 1545 and 1576, the years of the cocolitzli.

With the climate data in place, Acuña-Soto could piece together a convincing explanation of those epidemic years. Cocolitzli had been caused by a hemorrhagic fever virus that had lain dormant in its animal hosts, most likely rodents. Severe drought would have contained the population of rodents, forcing them to hole up wherever they could find water. Initially, only a small percentage may have been infected, but when forced into close quarters the virus was transmitted during bloody fights. Infected mother rodents then passed the virus to their young during pregnancy. When the rains returned, the rodents bred quickly and spread the virus—through their urine and feces—as they came into contact with humans in fields and homes. Once infected, humans transmitted the virus to one another through contact with blood, sweat, and saliva.

The article ends with a frisson, and not a pleasant one:

Cortés and his soldiers defeated, enslaved, and murdered the Aztecs, but now it seems that cocolitzli, a disease brought about by a native virus, is what really finished them off. Today the Aztec kingdom exists only in museums and ruins, but the virus could still be out there. As Mexico enters into yet another period of severe drought, could the killer reemerge?

"I don't know," Acuña-Soto says, then hesitates. "I don't think so. Although we have much poverty now, we don't have the kind of poverty and poor nutrition that the Indians suffered then. But I keep in close touch with my colleagues at the public hospital, just in case."