[AngelsSix]

There seemed little reason to debate the nature of the plague: Even the Spanish admitted that European smallpox was the disease that devastated the conquered Aztec empire. Case closed.

Then, four centuries later, Acuña-Soto improbably decided to reopen the investigation. Some key pieces of information—details that had been sitting, ignored, in the archives—just didn't add up. His studies of ancient documents revealed that the Aztecs were familiar with smallpox, perhaps even before Cortés arrived. They called it zahuatl. Spanish colonists wrote at the time that outbreaks of zahuatl occurred in 1520 and 1531 and, typical of smallpox, lasted about a year. As many as 8 million people died from those outbreaks. But the epidemic that appeared in 1545, followed by another in 1576, seemed to be another disease altogether. The Aztecs called those outbreaks by a separate name, cocolitzli. "For them, cocolitzli was something completely different and far more virulent," Acuña-Soto says. "Cocolitzli brought incomparable devastation that passed readily from one region to the next and killed quickly."

After 12 years of research, Acuña-Soto has come to agree with the Aztecs: The cocolitzli plagues of the mid-16th century probably had nothing to do with smallpox. In fact, they probably had little to do with the Spanish invasion. But they probably did have an origin that is worth knowing about in 2006.

A portly man with a full, close-cut dark beard, Acuna-Soto is a devoted scholar of all things Mexican. As we maneuver our way through the crowded streets jammed with taco stands around the General Hospital of Mexico, which serves Mexico City's poor and where Acuña-Soto often visits when not teaching at the National Autonomous University of Mexico, he effuses about everything from pre-Hispanic Mexican history to the quality of street-vendor tacos ("stay away from the salsa; it's got nearly as much bacteria as human feces"). When he was younger and thinner, Acuña-Soto says—before he went to Harvard University to study epidemiology and molecular biology—he interned as a physician in rural Chiapas, traveling by burro to patients in remote mountain villages.
On our drive south to his home in Cuernavaca, he recalls how his life changed after his return to Mexico in 1984. "When I came back here from Harvard, there was a big devaluation of the peso. My grant proposals had been accepted, but there was no money."

What might a restless epidemiologist do? With an eye toward writing an encyclopedia of Mexican diseases, Acuña-Soto began combing Mexico City's archives, researching the most famous epidemics, those that came after the Spanish conquistadores arrived.
The Aztec kingdom then was the last in a line of Mesoamerican states that emerged, flourished, and then vanished over the course of 2,500 years. Borrowing from the preceding Olmec, Teotihuacán, Mayan, and Toltec traditions, the Aztecs studied science and cosmology, agriculture, engineering, art, even archaeology. They had no written language but, using colorful and evocative pictographs, kept voluminous records in books of animal skin, agave fiber, or bark paper. Most of these, evoking ritual blood sacrifices, horrified the Spanish, who set about destroying the kingdom's library.

Fortunately for Acuña-Soto, some Spanish priests worked with the Aztecs to recapture their history, language, and culture before it was lost. In volumes of often colorful codices, key cultural and natural events in their lives were recalled and redrawn. Droughts, snows, and floods, good and bad harvests, all were re-created on the pages of these codices along with the changing geopolitical landscape.

The census data from the time of the Spanish invasion were so good that Acuña-Soto found he could track the movement of epidemics from village to village across the country. Friar Juan de Torquemada, a Franciscan historian writing in 1577, described the wake of cocolitzli in typical detail:

It was a thing of great bewilderment to see the people die. Many were dead and others almost dead, and nobody had the health or strength to help the diseased or bury the dead. In the cities and large towns, big ditches were dug, and from morning to sunset the priests did nothing else but carry the dead bodies and throw them into the ditches. . . . It lasted for one and a half years, and with great excess in the number of deaths. After the murderous epidemic, the Viceroy Martin Enriquez wanted to know the number of missing people in New Spain. After searching in towns and neighborhoods it was found that the number of deaths was more than two millions.

Medical historians insisted that the cause of all this affliction could only have been a European disease. But Acuña-Soto says, "The more I read of the cocolitzli, the more I realized that the descriptions of the disease and its spread did not fit any recognizable epidemiological paradigms."

It made no sense, of course, that the Aztecs had invented a new name for smallpox. And Acuña-Soto noticed that previous researchers had to pick and choose among the disease reports to make them fit a diagnosis of smallpox or typhus. He also could not understand why Old World diseases would cause massive deaths 20 years and then 55 years after the arrival of the Spanish. "By this time," Acuña-Soto says, "those who survived the earlier epidemics would have had immunities or would have passed them on."

"Historians assumed it must have been smallpox; it must have been typhus," Acuña-Soto recalls. "But historians are not epidemiologists."