From the sixth through eighth century A.D., a terrifying illness called the Plague of Justinian gripped the Eastern Roman Empire. Like the Black Death pandemic that decimated Europe seven centuries later, this plague often caused death within a matter of days. New work by paleogeneticists at the University of Tübingen in Germany provides empirical evidence for an old historical theory: that the two pandemics were caused by the same pathogen. By comparing the genome of ancient Yersinia pestis, the bacterium responsible for the Black Death, with genetic data from hundreds of less virulent modern strains, the team reconstructed the pathogen’s genetic history, and found evidence suggestive of a significant Y. pestis outbreak corresponding to the Justinian plague. “Our results suggest that Y. pestis may have affected human populations several centuries before the Black Death,” says Kirsten Bos, the study’s lead author. “The Plague of Justinian seems like the best candidate for an earlier pandemic.”