Two of history's deadliest plagues, which swept across Europe hundreds of years apart, were caused by different strains of the same deadly microbe, scientists say.

The finding raises the possibility that a new strain of plague could infect humanity again in the future.

The Justinian plague struck in the sixth century and is estimated to have killed between 30 and 50 million people—about half the world's population at that time—as it spread across Asia, North Africa, Arabia, and Europe.

The Black Death struck some 800 years later, killing 50 million Europeans between 1347 and 1351 alone.

Both plagues were spread to humans by rodents whose fleas carried the bacteria.

View Images Rodents like this dormouse helped spread the plague. Photograph by Stephen Dalton, Minden Pictures/Corbis

"These strains of plague that are endemic in rodent populations all around the world [today] are just as deadly as the strains that caused the [earlier] pandemics," said Dave Wagner, an associate professor at Northern Arizona University in Flagstaff and one of the authors of the new study, published in the Lancet Infectious Diseases.

"So the potential for a [modern] pandemic are still there because the strains are there," he said.

Unearthing a Common Origin

Wagner and his team confirmed a link between the two plagues after isolating tiny DNA fragments of the bacterium responsible for the Justinian plague, called Yersinia pestis, from the 1,500-year-old skeletons of two victims who were buried in Bavaria, Germany.

Because the plague bacterium lives in the blood of its victims, the researchers focused on the skeletons' teeth. "There's a lot of blood vessels going into your teeth," said Wagner, who works in Northern Arizona University's Center for Microbial Genetics and Genomics. "So they're a good place to find [Y. pestis DNA]."

After extracting the ancient DNA and then repairing it, the scientists compared the DNA with that of other strains of the same bacterium, contained in a database.

"We took the Justinian strain and compared it to whole-genome sequences of 130 modern strains, and also the Black Death strain," for which others had done whole-genome sequences before, Wagner said.

The scientists found that the strain responsible for the Justinian outbreak was related to, but distinct from, all other known strains of Y. pestis, including the ones responsible for the Black Death and a third pandemic in the 19th and 20th centuries, which scientists think originated in China and then spread globally, including to the United States.

The strain behind the Justinian plague was an evolutionary "dead end" that didn't survive to the modern day.

Threat of Reemergence

Y. pestis is present in about 200 species of rodents around the globe, so it's possible the plague could reemerge in the modern world, the scientists say.

"It does and will continue to spill over into humans every so often," said study leader Hendrik Poinar, a molecular evolutionary geneticist at McMaster University in Ontario, Canada.

However, if it does strike again, it will likely not be as devastating as in the past centuries for two reasons, scientists say.

"First, we've greatly improved hygiene since the times of the great pandemics," Wagner said, "which has controlled rat populations in these large urban centers where you expect these pandemics to start.

"The other thing is we now have antibiotics," he said, "and plague is susceptible to every antibiotic."