The bacterium Yersinia pestis, which caused the disease known as the Black Death in the fourteenth century, has been identified in DNA samples taken from 19 skeletons of people who died in sixth-century southern Germany. It is thought that these people were felled by the Justinianic Plague, which killed more than 100 million people between the sixth and eighth centuries. Named for the Byzantine emperor Justinian I, the plague is thought to have contributed to the decline of the Roman Empire.
this was after the "fall of the Roman empire" aka the fall of the western Roman empire, with the sacking of the city of Rome a few centuries earlier.
longer discussion moved to my medical blog.