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During his reign, which lasted from 527 to 565, Byzantine Emperor Justinian I initiated military campaigns in Italy, North Africa and elsewhere in an attempt to reconstruct the fallen Roman Empire. His efforts at territorial expansion suffered a major setback, however, when plague began spreading throughout the Mediterranean region in 541. Contemporary historians claimed that, at its height, up to 10,000 people perished daily in Constantinople (present-day Istanbul), the Byzantine Empire’s capital. Even Justinian I purportedly came down with the disease, though unlike so many others, he recovered from it. For two centuries, plague continuously wreaked havoc in places as far away as Ireland, the Iberian Peninsula, the Sahara desert and Persia. Then, having helped usher in the so-called Dark Ages, the outbreaks suddenly stopped. “The Justinian plague came, flashed, was hugely deadly,” said Hendrik N. Poinar, an evolutionary geneticist at McMaster University who co-authored Tuesday’s paper. “But then it appears to have gone extinct.”

Plague nonetheless flared up again in the mid-1300s, killing an estimated 30 percent to 60 percent of Europe’s population and likewise devastating much of the Middle East and North Africa. This Black Death “struck such fear into the hearts of men and women that brother abandoned brother, uncle abandoned nephew, sister left brother, and very often wife abandoned husband, and—even worse, almost unbelievable—fathers and mothers neglected to tend and care for their children as if they were not their own,” Italian author Giovanni Boccaccio wrote at the time. He described victims suffering from groin and armpit swelling and dark skin blotches. Outbreaks continued on and off in Europe until the 1700s and in Egypt until 1844. An apparent descendent of the Black Death then started the Third Pandemic in the 1850s in China’s Yunnan province. From there, it spread to India, Hong Kong and even the United States, killing about 12 million people, including 172 in San Francisco and dozens more in Los Angeles, Seattle and a few places along the Gulf Coast.

In order to learn more about the plague’s evolutionary history—“the family tree, if you will,” said Dave Wagner, an associate professor at Northern Arizona University and a co-author of the paper—scientists removed teeth from a pair of roughly 1,500-year-old victims found at an early medieval cemetery in Germany. They then extracted Yersinia pestis DNA from traces of blood found in the dental pulp and mapped the whole genome of two strains. “It’s the oldest pathogen genome that’s been reconstructed to date,” Poinar explained. Upon comparing them with Black Death and modern strains, the scientists concluded that the Plague of Justinian strains were distinct.

Scientists do not yet know why the Plague of Justinian strains died out whereas the Black Death strains did not. It also remains unclear whether Yersinia pestis caused earlier so-called plagues, such as the Plague of Athens in 430 B.C. and the Antonine Plague of 165 to 180 A.D., or whether a different pathogen was to blame. “When you go back that far it’s really difficult to find the samples,” Wagner said. “But if we got the samples it would be fun to try and test that.” He would also like to reconstruct additional Justinian-era plague strains. Meanwhile, the disease persists in North America, South America, Asia and Africa, occasionally transferring over from rodents to humans. Last year, an outbreak killed over 30 people in Madagascar. Thanks to antibiotics and hygienic improvements, however, another worldwide pandemic seems unlikely.

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