A tooth like this one, found in a 1,500-year-old grave site near Munich, harbored enough blood in the leftover bits of dental pulp that researchers were able to sequence the DNA. In that blood, they found not only human genetic material, but that of a bacterium — Yersinia pestis, aka the plague. The interesting thing here is that the plague of 6th-century Munich is not the same plague that swept through Europe in the 14th century under the nickname Black Death. The discovery means that Y. pestis jumped from rat to flea to human on more than one occasion, producing plagues that are genetically distinct.
The researchers are linking this earlier plague with the Plague of Justinian, a 6th century pandemic that killed millions. There's still debate over whether the Plague of Justinian was caused by the plague, as in Y. pestis, or by something else, largely because descriptions of symptoms don't totally match up with later plague outbreaks and the death toll is much larger than what we see in plague outbreaks today. (Those facts also apply to the Black Death, which was different from the Plague of Justinian and different from modern plague outbreaks.) Based on this evidence, we can't really say much about the Plague of Justinian, for certain. There's nothing directly linking the Munich bodies to it, specifically. But, if the Plague of Justinian was caused by Y. pestis, and there has been more than one time Y. pestis jumped to humans, then the differences between those strains could help explain the differences between the plagues attributed to the bacterium. So that's cool.