Strains of the same plague have caused two of the deadliest pandemics in history. Now, new versions of the bacteria could spark future outbreaks, scientists warned.
Researchers found tiny bits of DNA in the teeth of two German victims killed by the Justinian plague about 1,500 years ago, the Associated Press reported.
The genome of the oldest bacteria known was reconstructed with those fragments.
A strain of Yersinia pestis, the same pathogen responsible for the Black Death that struck medieval Europe, was concluded to be the cause of the Justinian plague.
According to the AP, the study was published online on Tuesday in the journal, Lancet Infectious Diseases.
As the Justinian Plague spread across Asia, North Africa, the Middle East and Europe, it packed quite a punch as it is thought to have wiped out half the globe.
During the 14th century, the Black Death killed about 50 million Europeans in just four years, the AP reported.
"What this shows is that the plague jumped into humans on several different occasions and has gone on a rampage," said Tom Gilbert, a professor at the Natural History Museum of Denmark who wrote an accompanying commentary. "That shows the jump is not that difficult to make and wasn't a wild fluke."
Rodents whose fleas carry the bacteria usually spread the plague to humans, the AP reported.
"Humans are infringing on rodents' territory, so it's only a matter of time before we get more exposure to them," Gilbert said.
Still, he and other experts doubted a modern plague epidemic would be as devastating.
"Plague is something that will continue to happen but modern-day antibiotics should be able to stop it," said Hendrik Poinar, director of the Ancient DNA Centre at McMaster University in Canada, who led the new research.
The plague could potentially infect other animals or humans as it is carried by about 200 rodent species, he said.
"Poinar warned that if the plague transforms into an airborne version - which can happen if the bacteria reaches the lungs and its droplets are spread by coughing - it would be much harder to snuff out. That type of plague can kill people within 24 hours of being infected," the AP reported.
To try averting future human infections, Poinar said scientists need to sharpen their surveillance of plague in rodent populations.
"If we happen to see a massive die-off of rodents somewhere with (the plague), then it would become alarming," he said.
There are several thousand human cases of plague every year, most often in central and Eastern Europe, Africa, Asia and parts of the Americas, the AP reported.
