It's easy to forget just how far medicine has progressed. While we may worry about the spread of infectious diseases like avian flu or meningitis today, those pathogens have nothing on fearsome pandemics in the distant past like the Plague of Justinian, which killed over 100 million people from the 6th to 8th centuries AD and helped usher in the Dark Ages in Europe.

The precise cause of the Justinian Plague, named for the emperor who ruled the Byzantine Empire at the time, was fiercely contested among historians and epidemiologists for years.

Now, researchers at the Johannes Gutenberg University Mainz (JGU) in Germany claim to have settled the debate.

After analyzing DNA from the skeletal remains of 6th century plague victims, they determined that the Justinian Plague pathogen was the same as that of the Black Death that came to Europe eight centuries later-the bacteria Yersinia pestis.

Dr. Barbara Bramanti of the JGU's Institute of Anthropology had previously led groundbreaking research on the Black Death, definitively concluding in 2010 that Yersinia pestis bacteria caused the 14th century pandemic.

In this follow-up study, Bramanti joined an international team led by Dr. Michaela Harbeck of the State Collection for Anthropology and Palaeoanatomy in Munich in analyzing skeletal DNA retrieved from an early medieval graveyard called Aschheim in Bavaria, Germany.

438 individuals were buried in the Aschheim cemetery. The research team noticed that multiple burials were clustered together in the second half of the sixth century-an indication that those people died in rapid succession and were likely to have been victims of the Justinian Plague.

The researchers collected teeth from 19 skeletons that had been buried in clusters, and screened the tissue for bacterial DNA.

The results, published last week in the journal PLoS Pathogens, leave little doubt that Yersinia pestis caused the Justinian Plague.

The genetic analysis was detailed enough to allow the researchers to trace likely origins of the bacterial strain.

Like later strains that caused the Black Death and a third plague pandemic in the 19th and 20th centuries, the Justinian Plague Yersinia pestis strain probably originated somewhere in Asia, afflicting the center of the Byzantine Empire in present-day Turkey and then sneaking through the Alps to infect the Bavarian villagers who were eventually buried in Aschheim.

"It remains questionable whether at the time of the Byzantine Emperor Justinian only one strain or more were disseminated in Europe, as it was at the time of the Black Death," said Bramanti in a news release. The team's future research will continue to explore how the deadly pandemic spread.

Citation: Harbeck M, Seifert L, Hänsch S, Wagner DM, Birdsell D, et al. (2013) Yersinia pestis DNA from Skeletal Remains from the 6th Century AD Reveals Insights into Justinianic Plague. PLoS Pathog 9(5): e1003349. doi:10.1371/journal.ppat.1003349

Correction: This article previously stated that the project was led by Dr. Barbara Bramanti; it has been corrected to note that the lead author was Dr. Michaela Harbeck of the State Collection for Anthropology and Palaeoanatomy in Munich. Researchers from the Bundeswehr Institute of Microbiology and Northern Arizona University also collaborated on the project.