Solving the Mystery of an Ancient Epidemic

No one knows what caused the Plague of Athens in the 5th century B.C. One popular theory is Ebola—but to discover the source of an outbreak millennia after the fact, scientists need victims’ remains and a bit of luck.

In the summer of 430 B.C., a mass outbreak of disease hit the city of Athens, ravaging the city’s population over the next five years. In his History of the Peloponnesian War, the historian Thucydides, who witnessed the epidemic, described victims’ “violent heats in the head,” “redness and inflammation in the eyes,” and tongues and throats “becoming bloody and emitting an unnatural and fetid breath.” Patients would experience hot flashes so extreme, he wrote, that they “could not bear to have on [them] clothing or linen even of the very lightest description.” In the later stages of infection, the disease would end with “violent ulceration” and diarrhea that left most too weak to survive.

More than 2,000 years later, the Plague of Athens remains a scientific mystery. Thucydides’ account—the only surviving description of the epidemic—has been the basis for dozens of modern-day theories about its cause, including bubonic plague, cholera, typhoid fever, influenza, and measles. And in June, an article in the journal Clinical Infectious Disease suggested another answer: Ebola.

The article, written by the infectious-disease specialist Powel Kazanjian, is the latest in a string of papers arguing that Athens was once the site of an Ebola outbreak. The surgeon Gayle Scarrow first raised the suggestion in The Ancient History Bulletin in 1988. Eight years later, the epidemiologist Patrick Olson published a letter in Emerging Infectious Diseases, a journal of the Centers for Disease Control and Prevention, comparing the symptoms of the Athens plague to those of Ebola, which had broken out in the Democratic Republic of Congo (then Zaire) and Sudan in 1976. “The profile of the ancient disease,” he concluded, “is remarkably similar.”

But not everyone was on board with Olson’s theory. In a 1996 interview with the The New York Times, the epidemiologist David Morens argued that Thucydides wasn’t the most reliable source: Unlike his contemporary, Hippocrates, he wasn’t a physician, and many of the terms he used to describe the disease’s symptoms were ambiguous. For example, the ancient Greek phlyktainai could refer to either blisters or callouses. Noting Thucydides’ claim that the epidemic had originated “in the parts of Ethiopia above Egypt” (today’s sub-Saharan Africa), Morens also questioned how people with Ebola, a highly contagious and deadly disease, could make it all the way to Greece without dying along the way.

The duration of the Athens epidemic also presented another problem: At five years, it was much longer than any known Ebola outbreaks, the majority of which lasted less than a year. And finally, Morens asked, if Ebola had made it out of Africa millennia ago, why were there no other accounts of the disease re-appearing anywhere on Earth until 1976?

Unfortunately for both Olson and Morens, however, neither had a more concrete way to back up their arguments. Their efforts to identify the Plague of Athens, like all the other efforts before them, could only rely on the written record left by Thucydides, which made confirmation more or less impossible.

This, in a nutshell, is the challenge of ancient pathology: With DNA testing, it’s often possible to identify the cause of an epidemic that took place centuries or even millennia ago. Finding remains of those victims to test, though, is another story.

Sometimes, scientists get lucky. In 2001, for example, a mass grave was uncovered at a construction site in Vilnus, Lithuania. Based on uniform fragments found in the grave, the bodies were identified as belonging to soldiers in Napoleon’s army—somewhere between 2,000 and 3,000 of them, hurriedly buried during the retreat from Moscow. When a team of anthropologists examined dental pulp taken from the bodies, they found that around one-third of them had died of typhus, a finding confirmed by tests of dead lice found at the site (the disease is transmitted through lice). Researchers had long suspected that typhus had contributed to Napoleon’s eventual defeat, but because knowledge of the disease was scant during his lifetime, historical accounts alone had never been enough to confirm it.

For the Plague of Athens, it seemed like a similar turning point had arrived in 1994, when during excavations for a planned Athens metro station at Kerameikos, an ancient graveyard used from the early Bronze age through Roman times. The excavators uncovered thousands of previously undiscovered tombs—including a set of seemingly hurried, unceremonious mass burials dating to 430 B.C., the year of the Plague of Athens.

Control of the site was turned over from the construction company to the Greek Ministry of Culture, which handles the discoveries of ancient ruins. In 2000, archaeologists turned over three teeth found at the site to a University of Athens team led by Manolis Papagrigorakis, an orthodontist and professor of dentistry, for DNA testing. Examining the dental pulp found in the teeth, Papagrigorakis’ team ran tests for seven diseases that had previously been suggested by other scholars: plague, typhus, anthrax, tuberculosis, cowpox, cat-scratch disease, and typhoid fever. The only match they identified on all three teeth was with the pathogen for typhoid fever. The researchers published the findings from their analysis in the International Journal of Infectious Diseases in 2006.

Far from solving the mystery, though, Papagrigorakis’s team only muddled it further. In a letter to the editor in the same journal, zoologists from Oxford University and the University of Copenhagen argued that Papagrigorakis’s methodology was flawed because he failed to do a phylogenetic analysis (a way of examining evolutionary relationships) on the teeth. Using the DNA data published in Papagrigorakis’s study, they conducted their own phylogenetic analysis, concluding that the DNA of the tooth bacteria was related to, but not the same as, that of the pathogen for typhoid fever. “The Athens [DNA] sequence and typhoid would have shared a common ancestor in the order of millions of years ago,” they wrote.

The authors also suggested another possibility: that the DNA found in the teeth wasn’t from the Plague of Athens pathogen at all. “While we cannot exclude the possibility that the Athens sequence is a previously unidentified infectious agent,” they concluded, “it is quite reasonable to assume that the sequence is actually that of a modern, free-living soil bacterium, a possibility that could have been explored by extracting DNA from surrounding soil samples as additional negative controls.”

Papagrigorakis currently has a new study underway, using more modern techniques and a greater number of tooth samples, that he hopes will help to settle the debate. In the decade since he published his Athens study, advancements in DNA-sequencing technology have enabled scientists to answer a number of lingering questions about ancient epidemics, making new discoveries from very old tooth samples. In 2011, for example, scientists used teeth taken from bodies in one of London’s so-called “plague pits” to sequence the genome of the bacterium y. pestis, the source of the Black Death epidemic that had swept Europe in the 14th century. By comparing the old genome to modern-day strains, the researchers were able to reconstruct the bacterium’s evolutionary path over the centuries, finding support for the idea that the 14th-century pathogen was likely the root of the evolutionary tree leading to more recent outbreaks.

And in a 2014 study published in the Lancet Infectious Diseases, scientists were able to prove for the first time that the Plague of Justinian—which killed about 50 million people in Europe and the Byzantine Empire between 600 and 800 A.D.—was actually a strain of y. pestis, making it the first known outbreak. The team made its discovery by sequencing DNA from teeth taken from human remains that had been found in a German graveyard and dated to the time of the epidemic.

Even when ancient specimens are available, though, they may not be enough to identify a disease. Bacteria, like typhoid and plague, can be identified through DNA sampling, but this isn’t always the case with viruses. Many of them, including the viruses for Ebola, influenza, and measles, require an RNA sample for positive identification—and thus far, the oldest preserved RNA viral genome belongs to a 700-year-old specimen of caribou feces, much more recent than the Athens samples from in the 5th century B.C. The structure of RNA makes it much more unstable—and therefore more prone to degradation—than DNA, meaning that if the Plague of Athens was viral rather than bacterial, its source may remain a mystery.

“If Ebola virus was there, we will never know,” said Vinent Racaniello, a professor of microbiology at Columbia University professor and the host of the podcast This Week in Virology. “For that, we’ll need a time machine to bring us back to get samples.”

Partially due to these limitations, Kazanjian’s recent study doesn’t delve into dental-pulp analysis data. His argument is based on the similarity between the symptoms of the Plague of Athens and those of Ebola, an argument that he believes is strengthened by observations from the latest Ebola outbreak. The paper ends with a chart of the symptoms described by Thucydides, listed side-by-side against the symptoms of eight modern diseases that had previously been floated as possible explanations; of all of them, the symptoms for Ebola have the most overlap.

Even so, Kazanjian cautioned against referring to Ebola as a “probable” or even a “likely” cause. “The most accurate statement is that the cause remains unknown, and there are several possibilities,” he said, including that the Plague of Athens may have been a now-extinct disease with Ebola-like symptoms.

He also acknowledges the difficulty of making rigorous comparisons between Thucydides’s descriptions and modern-day medical knowledge: “I try not to get into the trap of saying what the most likely thing is,” he said.

But for Kazanjian—also a historian—solving the puzzle of the Plague of Athens is less compelling than exploring all the possibilities. The inquiry is “clearly fun to do,” he said, “no matter what your background is.”