PARIS (AFP) ― “In some cases death came immediately; in others, after many days,” the historian Procopius wrote as a terrifying disease scythed through Constantinople in 542.

“With some, the body broke out with black pustules about as large as a lentil and these did not survive even one day, but all succumbed immediately. Vomiting of blood ensued in many, without visible cause, and immediately brought death.”

What Procopius observed first hand was the Plague of Justinian, named after the Eastern Roman emperor he served and who contracted the disease but survived.

The first of three plague pandemics to have ravaged humanity, it killed between 25 and 100 million people across Asia, North Africa and Europe.

After an initial two-year rampage, it returned in waves before mysteriously disappearing in the middle of the eighth century.

Using DNA teased from 1,500-year-old teeth of plague victims buried in Germany, scientists have reconstructed the genetic profile of the killer and say its ability to mutate is a warning for people today.

The strain of Yersinia pestis germ which caused the Plague of Justinian was different from the strain that triggered the Black Death in the 14th century, killing an estimated 30 million Europeans, they found.

It is also distinct from the Y. pestis strain that caused a third outbreak of plague in the late 19th century, and which was likely to have been a genetic offshoot from the Black Death microbe.

The evidence confirms, as expected, the role of rats as the germ’s “reservoir” or natural source, according to a paper published in the journal the Lancet Infectious Diseases.

It also throws up new avenues for exploring the dynamics of plague: why a pandemic erupts and dies out, and how a novel strain emerges, becoming a threat against which the immune system has no defense.

“We know the bacterium Y. pestis has jumped from rodents into humans throughout history, and rodent reservoirs of plague still exist today in many parts of the world,” said Dave Wagner, an associate professor in the Center for Microbial Genetics and Genomics at Northern Arizona University.