To help solve this mystery, scientists investigated ancient DNA from the teeth of 19 different sixth-century skeletons from a medieval graveyard in Bavaria, Germany, of people who apparently succumbed to the Justinianic Plague.
They unambiguously found the plague bacterium Y. pestis there.
"It is always very exciting when we can find out the actual cause of the pestilences of the past," said researcher Barbara Bramanti, an archaeogeneticist at Johannes Gutenberg University in Mainz, Germany.
"After such a long time -- nearly 1,500 years -- one is still able to detect the agent of plague by modern molecular methods," researcher Holger Scholz, a molecular microbiologist at the Bundeswehr Institute of Microbiology in Munich, Germany, told LiveScience.
The researchers said these findings confirm that the Justinianic Plague crossed the Alps, killing people in what is now Bavaria. Analysis of the DNA suggests that much like the later two pandemics of plague, this first pandemic originated in Asia, "even if historical records say that it arrived first in Africa before spreading to the Mediterranean basin and to Europe," Bramanti told LiveScience.