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Scientists Think They've Figured Out What Caused The Horrific Eye-Bleeding Disease That Killed 80% Of The Aztecs


Tom Hale


Tom Hale

Senior Journalist

Tom is a writer in London with a Master's degree in Journalism whose editorial work covers anything from health and the environment to technology and archaeology.

Senior Journalist

An estimated 80 percent of the population was wiped out by this epidemic. Original illustration by unknown 16th-century artist. Published in Pestilence and Headcolds: Encountering Illness in Colonial Mexico. Wikimedia Commons/Public Domain

In 1545 CE, a mysterious illness crashed through Mexico, causing one of the biggest epidemics of human history. Within just five years, up to 15 million people – about 80 percent of the population – died a gory death after suffering from bleeding of the eyes, nose, and mouth.

Some 550 years on, a team of German scientists has struck upon a potential culprit behind the so-called "cocoliztli" (meaning pestilence in the Aztec language) epidemic: we're looking at you, Salmonella. Their study about this puzzling pathogen is published in Nature Ecology and Evolution,


The cocoliztli epidemic started in Mexico after Spanish colonizers came into contact with the native population. Since the native people had not developed immunity to many of the "Old World" diseases, the disease spread like wildfire and managed to wipe out an estimated 80 percent of the population.

Historians had previously postured that the cocoliztli epidemic could have been caused by one of the many other diseases the Europeans brought across the Atlantic, such as smallpox, measles, or typhus. Some have even suggested it could have been an Ebola-like virus. However, it’s particularly hard for scientists to identify viruses from the past, as their symptoms and genetic makeup can change significantly over the centuries.

Using a new method of pathogen identification, researchers have now looked for DNA from the teeth of 29 disease victims buried at a cemetery in Teposcolula-Yucundaa, Mexico. This analysis showed evidence of the Salmonella enterica bacterium of the Paratyphi C variety, an infectious bacteria that causes high fever, rashes, severe headaches, and other typhoid-like symptoms. That means that this study is also the very first time that scientists have found molecular evidence for this microbial infection by analyzing old material from the New World.

"This new approach allows us to examine skeletons in broad-based investigations for all pathogens that may have been present in them," Johannes Krause, director of the Department of Archaeogenetics and professor at the University of Tübingen in Germany, explained in a statement.


This is by no means a closed case, the researchers conceded, as the presence of Paratyphi C in a few dozen skeletons doesn't necessarily mean that 15 million people died from it. Furthermore, this diagnosis doesn't quite explain the horrific bleeding that many sources documented. Nevertheless, their research has proven to hold some real potential for unraveling the secrets of past epidemics in history.

"This is a significant advance in the methods we have to research on past diseases. We can now check the presence of numerous infectious organisms in archaeological material,” added Kirsten Bos of the Max Planck Institute. “This is especially relevant in cases where the cause of a disease was previously unknown."


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