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When people hear the words bubonic plague, they usually think of the Black Death, the epidemic that hit Europe in the 14th century. You learn about this in school. But the first recorded plague pandemic actually happened 800 years earlier, during the days of the Roman Empire.

NPR's Nell Greenfieldboyce reports that scientists have now recovered the entire genetic code of the bacteria responsible for that ancient pandemic.

NELL GREENFIELDBOYCE, BYLINE: This plague struck in the year 541, when the Roman emperor was named Justinian, so it's usually called the Justinian Plague. He actually got sick himself, but recovered. He was one of the lucky ones.

DAVID WAGNER: Some of the estimates are that up to 50 million people died.

GREENFIELDBOYCE: David Wagner is an expert on microbial genetics at Northern Arizona University. He says the plague swept through Europe, North Africa and parts of Asia.

And it's thought that the Justinian plague actually led partially to the downfall of the Roman Empire.

Contemporary accounts of this plague made scientists suspect it was caused by the bacteria Yersinia pestis, the same kind of microbe that later caused Europe's Black Death. The bacteria gets spread by fleas. After someone gets infected from a flea bite, the bacteria travel to the nearest lymph node and start multiplying.

WAGNER: And so you get this mass swelling in that lymph node, which is known as a buboe. That's where the term bubonic plague comes from.

GREENFIELDBOYCE: The Justinian plague has been hard to study scientifically, since it happened around 1,500 years ago. But recently new housing developments being built outside of Munich led to some old farmland being dug up and people uncovered a burial site with graves that dated as far back as the Justinian Plague.

WAGNER: They found some that had multiple individuals buried together, which is oftentimes indicative of an infectious disease. And so in this particular case we examined material from two different victims. One of those victims was buried together with another adult and a child, so it's presumed that they all may have died of plague at the same time.

GREENFIELDBOYCE: All that was left was skeletons - with teeth. Inside the teeth was the dental pulp that still contained traces of blood. And the blood contains the remains of plague bacteria. Wagner and an international team of collaborators managed to extract enough DNA to reconstruct the ancient pathogen's entire genetic code.

In the journal Lancet Infectious Disease, the team says it looks like this strain of bacteria jumped from rodents into humans and then died out. The later emergence of the Black Death seems to have been caused by a separate event. The DNA also suggests that like the Black Death, the original source of the plague was China.

PAUL KEIM: So the ecological reservoir for plague, the historical reservoir, is in China and it's this emergence, this pattern over and over again, with people moving commodities, rats, and fleas around the world, that we're able to document.

GREENFIELDBOYCE: Paul Keim worked on the study at Northern Arizona University. He says overall this ancient strain is not that different from modern ones that still circulate in places like Arizona.

KEIM: The biology of the pathogen no doubt could cause another pandemic if it weren't for the changes in human culture and medicine.

GREENFIELDBOYCE: He says these days antibiotics quickly stop plague outbreaks in their tracks.

Nell Greenfieldboyce, NPR News.

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