Climate, Not Spaniards, Brought Diseases That Killed Aztecs

By Guest Blogger | August 1, 2013 8:00 am

By Linda Marsa

The following excerpt from Marsa’s forthcoming book, “Fevered: How a Hotter Planet Will Harm Our Health and How We Can Save Ourselves,” was originally published on PLOS Blogs as part of their series “The Science of Extinction and Survival: Conversations on Climate Change.”

The wild swings in weather that are expected to become commonplace as the planet gets warmer—more frequent and severe droughts, followed by drenching rains—change ecosystems in a way that awaken and expedite the transmission of once dormant diseases.

Intriguingly, this type of weather pattern may be what led to the fall of the once mighty Aztec Empire in the early 16th century–and not as is commonly held, by the invasion of European colonialists, who brought with them diseases like mumps, measles and smallpox for which the native populations lacked immunity.

Curious timing

When Hernando Cortes and his army conquered Mexico starting in 1519, there were roughly about 25 million people living in what is now Mexico. A hundred years later, after a series of epidemics decimated the local population, perhaps as few as 1.2 million natives survived. Records confirm there was a smallpox epidemic in 1519 and 1520, immediately after the Europeans arrived, killing between 5 and 8 million people. But it was two cataclysmic epidemics that occurred in 1545 and 1576, 25 and 55 years after the Spanish conquest, which swept through the Mexican highlands and claimed as many as 17 million lives.

To Dr. Rodolfo Acuna-Soto, a Harvard-trained infectious disease specialist at the National Autonomous University of Mexico in Mexico City, it made no sense that a deadly outbreak of European origin could occur so long after the Spanish arrived, because the natives who survived previous plagues would have passed on their immunities.

To find answers, Acuna-Soto spent a dozen year poring through ancient documents written by 16th century Spanish priests who worked with the Aztecs to preserve a record of their history, language and culture. These texts also tracked key natural events—storms, droughts, frosts and illness. In particular, they detailed the plagues of cocoliztli (Nahuati for “pest”), a disease that seemed far more virulent than smallpox.

The Aztecs’ mysterious plague

“Nobody had the health or strength to help the diseased or bury the dead,” one Franciscan friar wrote in 1577 about the devastation from cocolitzli. “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.”

Acuna-Soto also had access to exhaustive diaries kept by Francisco Hernandez, the surgeon general of New Spain who witnessed the second catastrophic epidemic in 1576. He described a highly contagious and lethal scourge that killed within a few days, causing raging fevers, jaundice, tremors, dysentery, abdominal and chest pains, enormous thirst, delirium and seizures. “Blood flowed from the ears,” the physician observed, “and in many cases blood truly gushed from the nose.”

“These symptoms didn’t sound like smallpox or any other known European disease that was in Mexico during the 16th century,” Acuna-Soto told me. “This sounded like a hemorrhagic fever. So if the Spanish didn’t bring about the fever, what did?”

Querying the climate

In his research, Acuna-Soto had noticed a pattern: the plague was preceded by years of severe drought but the epidemics occurred only during wet weather, and heavy rainfall. To confirm his observations, Acuna-Soto worked with a Mexican-American team of dendrochronologists—scientists who study tree rings to date changes in climate—and compared the 16th century historical accounts with tree-ring records from a forest of 450-year-old Douglas fir trees in a remote region of central Mexico near Durango.

The tree rings indicated that the most severe and sustained drought in North America in the past 500 years occurred in the mid-16th century. But there were heavy downpours in the years around 1545 and 1576, which coincided with the cocolitzli outbreaks. “The smoking gun was the tree ring data,” Acuna-Soto said.

Acuna-Soto is now convinced that the death knell for the Aztecs was an indigenous hemorrhagic fever virus spread by rodents, not the Spanish conquest.

The rat population was depleted during the drought, when food was scarce. But once the rains returned, food and water were suddenly plentiful and the number of infected rats exploded, spreading the deadly scourge to humans.

As weather swings become more erratic and the Southwest bakes under increasingly prolonged droughts, epidemics like cocolitzli will doubtless return. “Sooner or later, a new virus will emerge from the desert for which we don’t have any vaccine and we can’t treat with drugs,” said Acuna-Soto. “That’s guaranteed. That’s the big fear of science. The only thing we don’t know is when.”

For more information on Linda Marsa’s new book, visit her website.

Top image by Kim Alaniz via Flickr


1. Rodolfo Acuna-Soto, David W. Stahle, Malcolm K. Cleaveland, Matthew D. Therrell (2002). Megadrought and Megadeath in 16th Century Mexico. Emerging Infectious Diseases. 2002 April; 8(4): 360–362.

2. Acuna-Soto, R., Romero, L., & Maguire, J. (2000). Large Epidemics of Hemorrhagic Fevers in Mexico 1545-1815. The American Society of Tropical Medicine and Hygiene, 62 (6), 733-739.

CATEGORIZED UNDER: Health & Medicine, Top Posts
MORE ABOUT: aztecs, climate change
  • J.S.

    How was the Spanish population living in Mexico affected by the epidemic of 1576-77?

    • BeanSoupMagyar

      That would be interesting to know. Even if they were not as badly affected though- it doesn’t mean a genetic ability to fight the disease that Aztecs had not.
      The Spaniards would doubtlessly have been wealthier- with better access to food, medical care, cleaner conditions and less cramped living. That alone could have potentially made the death toll lower on the Spaniards.

      • Miranda Waters

        With hemorrhagic fevers transmitted through rodents, the only real way to avoid sickness is to avoid rodents. More specifically, their excrement. With this in mind, cleaner conditions is really all you need to avoid catching cocoliztli. If, this is in fact what was going on. I too am curious as to the toll on the Spanish.

        • gwyllion

          actually their urine. I was working in the epidemiology dept at UNM the year haunta made its appearance there. No one knew what it was – international news crews showed up at the University hospital – rumor was a war grade virus had escaped from the research center at four corners – it was terrifying!!

  • JonFrum

    Bwahahaha! Stop – you’re killing me! It used to be the gods sending pestilence to punish sinning populations. Now it’s Gaia punishing us for our climate sins.

    • Miranda Waters

      “Gaia punishing us for our climate sins”… you didn’t seriously take that away from this article right?

      • OWilson

        Same old time worn religious message though.
        “The end is nigh”
        “Give up your wordly goods (SUVs, taxes) and ye shall be saved”
        Second oldest profession.

        • DariusPicard

          Whoring is still whoring. You are still screwed for money.

  • Jose Hidalgo

    So so sad that this article doesn’t get the names right… It is the first time I know about Hernando Cortés.

    • jaimjackson

      The name is correct as it stands, Hernando Cortés. It is also correctly rendered Hernán Cortéz. Just a spelling variation.

      • Jose Hidalgo

        You are wrong, Hernando doesn’t exist. If I hadn’t lived in central america I could think you may be right, but Hernando doesn’t exist either in Central America or Spain. The correct name is Hernán.

        • jaimjackson

          Maybe it’s a regional thing. I had two students named “Hernando” (one Mexican, one Costa Rican) and I know Hernando is also a place-name (Florida, Mississippi). It’s very ,very common for a given name to have numerous spelling variants, derivatives and diminutives. Check “Hernando Cortes” in Google and you’ll get over a hundred thousand results.

  • John Shuey

    So it was the Spaniards driving all over the New World in their SUVs and burning coal in their power plants that caused such dramatic climate conditions?

  • Oscar

    Please excuse my English, I’m not very fluent in it.

    But how many spaniards where killed during this outbrake? wasn’t the disease transmited from human to human? If so, what was the porcentage of spaniards vs natives?



  • James T. Richmond

    We may not know what the disease will be, but we do know how it will spread: rodents. I say we get some smart people devoted to creating a way to quell major influxes of rodent populations in order to prevent outbreaks. Any ideas?

  • Guest

    Why didn’t the fever go back across the Atlantic and depopulate Europe? Plenty of rodents there, and on the ships going back and forth?

  • Eric Mankin

    But why didn’t the fever cross back to Europe and kill millions there. No shortage of rodents, either in Eurasia or on the boats going back and forth. Also, the indigenous mass depopulations in the New World were not at all limited to Mexico or a short 16th century time window, as _1491_ and earlier, _Guns, Germs and Steel_ have discussed in detail.

    • PorkChop

      The Europeans had developed immunity to these diseases, after being exposed to the bacteria for dozens of centuries. The Aztecs had no immunity to these new bacteria, which they had never been exposed to until the Spanish arrived.

      • jlk7e

        The Europeans had developed immunity to smallpox. This supposedly is some sort of indigenous hemorrhagic fever, which Europeans would have had no greater immunity to than Indians.

        • Eric Mankin

          And diseases did go from the Americas to Europe: Exhibit A is syphillis, (or used to be: its American origin is now questioned).


        You’re speaking in a very general way, but immunity is very specific. There’s no reason to think immunity to a bunch of other things would protect Europeans from this.

        In fact, Americans undoubtedly were exposed to countless pathogens over the millennia, and yet that didn’t help them against small pox. It was luck of the draw, and they could just as easily given us some apocalyptic pathogen that killed off 98% of Europe.

        The article presents an intriguing hypothesis, but I hasn’t really nailed the thing down.

    • Newcastle

      Hemorrhagic fevers can have very specific hosts. Not just any mouse or rat will do. Hantavirus in the southwest US is a good example. It is caused by the Sin Nombre virus and is carried by deer mice. Not by rats, not by house mice. Luckily for us deer mice prefer to live away from people so the disease it not common. The vast majority of rodent species do not hitch rides on ships and travel the world.

      • Eric Mankin

        I hear you. But this one seems to have been carried by a common rodent that frequented human habitations. Apparently this rodent whatever it was didn’t hop a ship – but, again it would be good to figure out the chain…

    • Michael Vranian

      Hemorrhagic fever kills too quickly to spread across the ocean in the 16th century. If a ship were infected it would likely never reach its destination.

      • Eric Mankin

        Maybe. But even a ship down to a skeleton (the traditional word) could have made the crossing.

    • Felipe Casas

      You are correct, why werent the spanish affected? Or the african slaves residing in Mexico? Or any other eurasian at that time? Good question

      • Eric Mankin

        The African slaves brought their own set of African diseases, including yellow fever and highly virulent new strains of malaria…

  • Bill Koury

    When the next severe drought period ends and good times return, I’ll be
    sure to remember this example and set out plenty of rat traps!

  • JJKM

    Interesting summary of the CDC article, but you left out a critical part:

    “These infections appear to have been aggravated by the extreme climatic conditions of the time and by the poor living conditions and harsh treatment of the native people under the encomienda system of New Spain. The Mexican natives in the encomienda system were treated as virtual slaves, were poorly fed and clothed, and were greatly overworked as farm and mine laborers. This harsh treatment appears to have left them particularly vulnerable to epidemic disease.”

  • PorkChop

    Oh yes, now they should erect hospitals named after the mighty Spanish explorers, for all of the benefits they brought to the “new World”. Why would anyone imagine the diseases could have come from the Spanish, after never existing in this region until they arrived?

  • bobito

    I hope there is a chapter in the book titled “However, I could be completely wrong about this.”

    Shame on Discover for the matter of fact title to this article, all they had to do is add “Did the” at the front, and a “?” at the end. Is Discover trying to imply there is scientific consensus on this?

  • SocraticGadfly

    Phrases like “compelling narrative” and ” has been largely ignored by governments and policy makers” … make me leery of an oversell. Plus, as a commenter on her blog noted, there IS a hemorrhagic version of smallpox. The book sounds like an oversell.

  • alanborky

    Ah science it’s supposedly all about objectivity yet how prone to fashion it is.

    So during the Cold War period when we were all trained to fear nuclear winter scientists wailed all through the Sixties into the Seventies “Woe to the coming Ice Age!”

    And now destabilisation of the climate leading to Global Warming’s the big thing so of course we get rewrites of history.

    Of course this rewrite might be true but I did Environmental Science at University and actually investigated this very subject of what effect climate may’ve played on how easily various South American tribes’ succumbed to the Spanish.

    Me own personal conclusion was their complete and catastrophic loss of their centuries old lifestyles and beliefs as a result of total defeat led to massive depression and a lethal impairment of their immune systems’ abilities to handle both European diseases and local environmental setbacks.

    The Spanish on the other hand were subject to an entire continent’s worth of unknown diseases as well as exactly the same environmental setbacks as the natives yet their overwhelming victories plus the fact they were the ones imposing the culture seemed to enhance and strengthen their immune systems.

    • The Energizer

      Nonsense. The only source for the so called “ice age” argument was a poorly researched article in Time magazine. The earth’s atmosphere has topped 400 ppm of CO2, the so called “tipping point” of anthropogenic climate change. We shall bear witness to that which has never occurred in human history.

  • Arnold Karpoff

    I suppose the weather brought Syphillis to Europe.

    • DariusPicard

      Heather, it was Heather.

  • DariusPicard

    Sounds like the planet might finally be rid of the parasite which has inflicted it for eons.

  • Joseph

    I did not see anything about whether the Spanish also succumbed to the disease. I would assume they would not have immunity to a disease that originated in the Americas.


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