Ever heard of cocoliztli?
In one way, it's shocking if you haven't, and in another, hardly surprising at all, because the vast majority of its victims were the indigenous people of Mexico and Central America, and history has a way of ignoring what happened to brown-skinned people. Cocoliztli is the Nahuatl name for a contagious, usually fatal disease that struck Mesoamerica repeatedly, with the worst recorded outbreaks in the sixteenth century, killing an estimated ten million people. This puts it in fifth place for the worst pandemics known, after the Black Death (estimated one hundred million casualties), Justinian's plague (fifty million), HIV/AIDS (forty million), and the Spanish flu (thirty million). [Nota bene: if we're adding up total death toll, one of the worst is smallpox, but as that was endemic and widespread, I'm not counting that as a true pandemic. In eighteenth-century Europe, for example, it's estimated that four hundred thousand people died of smallpox per year; and its introduction into the Americas decimated Native populations. It's likely we'll never know for sure how big the death toll was, but it was huge.]
The symptoms of cocoliztli were awful. Severe headache, high fever, vertigo, jaundice, and abdominal cramps. The worst was the hemorrhaging -- victims bled from every orifice including the tear ducts. Most of the victims died, usually between four and seven days after onset.
There are two curious things about cocoliztli. The first is that there hasn't been a confirmed case of it since 1813.
So where has it gone? Ordinarily, infectious diseases occur at low rates until a confluence of events triggers a more widespread outbreak. Consider, for example, the Black Death. Bubonic plague (caused by the bacteria Yersinia pestis) has been present in humans for millennia, but a perfect storm occurred in the mid-fourteenth century that caused the most devastating pandemic in history. First, it was the beginning of the Little Ice Age, and the lower temperatures drove rats (and the fleas they carried) indoors, and into contact with humans. Second, trade throughout Europe, and with Asia (via the Silk Road), had really just started to gear up, and rats are notorious for stowing away on ships. And third, the population had risen -- and larger, more crowded cities facilitate disease spread.
Cocoliztli, though, hit Mesoamerica hard, and seemingly out of nowhere. Repeated outbreaks in 1545, 1576, 1736, and 1813 killed millions, but in between, we don't know where it went -- or why after 1813 it apparently vanished completely.
The second odd thing is that we still don't know what caused it.
The bones of presumed victims have offered up only debatable information. Back in 2018, Johannes Krause, of the Max Planck Institute for the Science of Human History, found DNA in bones from victims of the 1545 outbreak that seems to come from a Salmonella enterica strain called Paratyphi C, but that doesn't mean that's what killed them -- and one epidemiologist has pointed out that typhoid fever, which is caused by S. enterica, doesn't have the same symptoms as cocoliztli. Others suggest that its symptoms are more consistent with a viral hemorrhagic fever like Ebola, Lassa, and Marburg, but there are no viruses known that are endemic to the Americas and cause symptoms like that.
A rather sobering possibility is that the pathogen, whatever it is, resides in an animal vector -- that is, it's a zöonotic disease, one that exists in an animal population and is reintroduced to humans periodically upon contact. If so, it's unknown what that vector might be -- but the jungles of Central America are a big place, and there are lots of animals there in which a pathogen might hide.
Whatever causes it, and wherever it went, it's to be hoped it's gone for good. This would put it in the same class as the mysterious European sweating sickness, that caused repeated outbreaks in the late fifteenth and early sixteenth centuries, and then vanished, apparently permanently. It, like cocoliztli, was highly infectious -- but the pathogen remains unidentified.
Cocoliztli left its mark on history. The population of Mexico collapsed in the sixteenth century, largely due to the outbreaks, dropping from an estimated twenty-two million in 1500 to two million a hundred years later. This undoubtedly contributed to the Spanish takeover -- something that reverberates to the present day.
It's also an enduring mystery. How such a virulent disease could strike so hard, decimating an entire region, and then vanish utterly is bizarre. But it does highlight how important epidemiological research is -- helping us to understand how pathogens cause disease, and how they jump from one host to the other. Giving us, it is to be hoped, the tools for stopping the next pandemic before it happens.
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