The medieval Silk Road brought a wealth of goods, spices, and new ideas from China and Central Asia to Europe. In 1346, the trade also likely carried the deadly bubonic plague that killed as many as half of all Europeans within 7 years, in what is known as the Black Death. Later outbreaks in Europe were thought to have arrived from the east via a similar route. Now, scientists have evidence that a virulent strain of the Black Death bacterium lurked for centuries in Europe while also working its way back to Asia, with terrifying consequences.

At the Society for American Archaeology meetings earlier this month in Orlando, Florida, researchers reported analyzing the remains of medieval victims in London; Barcelona, Spain; and Bolgar, a city along the Volga River in Russia. They determined that the victims all died of a highly similar strain of Yersinia pestis, the plague bacterium, which mutated in Europe and then traveled eastward in the decade following the Black Death. The findings “are like pearls on a chain” that begins in western Europe, said Johannes Krause at the Max Planck Institute for the Science of Human History in Jena, Germany, an author of a soon-to-be-published study. (The lead author is Maria Spyrou, also at Jena.)

That chain may have stretched far beyond Russia. Krause argues that a descendant of the 14th century plague bacterium was the source of most of the world’s major outbreaks, including those that raged across East Asia in the 19th and 20th centuries and one afflicting Madagascar today. Eric Klingelhofer, an emeritus archaeologist at Mercer University in Macon, Georgia, called Krause’s presentation “a good piece of research.” But molecular microbiologist Holger Scholz at Munich, Germany’s Bundeswehr Institute of Microbiology is skeptical. “I just think it’s not very likely that a strain from China came to Europe, survived there for a couple of hundred years, and moved back to China,” she said. “That sounds pretty adventurous.”

Advances in sequencing the DNA of pathogens found in ancient human skeletons are driving new research—and debate—on the spread of plague. Thanks to a series of recent findings, the notion that plague remained in Europe for centuries after the Black Death, rather than arriving in repeated waves from Asia as historians long assumed, is gaining ground.

A team led by Lisa Seifert at Munich’s Ludwig Maximilian University reported in January that the Black Death strain persisted in Europe for at least 3 centuries, based on DNA sequences from eight skeletons at two burial sites in Germany that spanned the 14th to the 17th centuries. The sequences were “highly similar” to those from earlier European victims, according to the study, which included Scholz. While not precluding continued waves of plague coming from Asia, the team concluded that there was “a long-time persistence of the pathogen in a not-yet-identified reservoir”—perhaps rats.

Also in January, a team led by Kirsten Bos at Jena’s Max Planck Institute reported further evidence that a descendant of the Black Death strain hung on in Europe, implicating it in the last major European plague outbreak, in Marseille, France. Using DNA from the teeth of five individuals who died in 1722, the group found that the Y. pestis strain in Marseille likely evolved from the Black Death. “Our results suggest that the disease was hiding somewhere in Europe for several hundred years,” said Bos, whose team included Krause.

Now, Krause has traced the Black Death’s eastward spread. His team studied skeletons from a cemetery near the Tower of London firmly dated to 1348–1350, in the wake of the Black Death, as well as from a Barcelona cemetery radiocarbon-dated to the mid-14th century. The Russian evidence comes from a site that included coins from 1360; the burial is estimated to have taken place between the early 1360s and 1400. DNA sequencing from all three places revealed the same strain of Y. pestis. This strain appears to be the ancestor of the one that killed millions in 19th century China, based on phylogenetic clues.

“If the plague in China was actually European in origin, it’s a cruel irony of history,” says Klingelhofer, who notes that this was the era when Western powers dominated China. Krause adds that the plague affecting Madagascar as recently as last year also seems genetically related to the variety that spread east from Europe in the 14th century.

Researchers are eager to create a plague family tree in order to understand the movements and impact of different varieties of Y. pestis across time and space. Krause argues that three of the four branches of plague seem to have evolved in Asia. But he says the branch related to the strain that developed in Europe immediately after the Black Death has proved the most mobile and devastating.

Krause admits that between 14th century London and 21st century Madagascar, there are “a lot of steps missing” to identify the precise movements of the deadly bacterium. But he says that understanding plague’s long journey could help researchers limit its future spread.