The discovery that the Black Death and Justinian Plague were caused by different strains of the same pathogen suggests a new strain of plague could emerge again, researchers warn.

Hendrik Poinar, an associate professor and director of the McMaster Ancient DNA Center, called the new study "fascinating and perplexing," saying "it generates new questions which need to be explored, for example why did this pandemic, which killed somewhere between 50 and 100 million people die out?"

The Justinian Plague, which took place during the sixth century, is believed to have killed off between 30 million and 50 million people, or half of the world's population. Exactly how the deadly disease was related to the Black Death, which struck roughly 800 years later and with similarly deadly force, has eluded scientists.

To find out, Poinar and his colleagues isolated DNA fragments from the teeth of two victims of the Justinian Plague. Using these fragments, they were able to reconstruct the genome of the oldest Yersinia pestis - the bacterium behind the plague, which they then compared to the genomes of more than 100 contemporary strains.

The results indicate that the strain behind the earlier outbreak was an evolutionary "dead end" and distinct from the strains responsible for the Black Death and other plague pandemics that came after.

"We know the bacterium Y. pestis has jumped from rodents into humans throughout history and rodent reservoirs of plague still exist today in many parts of the world," said Dave Wagner, an associate professor in the Center for Microbial Genetics and Genomics at Northern Arizona University. "If the Justinian plague could erupt in the human population, cause a massive pandemic, and then die out, it suggest it could happen again," he explained.

However, Wagner notes, all is not necessarily lost.

"Fortunately we now have antibiotics that could be used to effectively treat plague, which lessens the chances of another large scale human pandemic."

The results were published by the journal The Lancet Infectious Diseases.