Could the Black Death possibly strike again? Scientists now warn that the murderous bubonic plague which consumed nearly 100 million's lives could return.

The scientists genetically linked two of the deadliest pandemics in history -- The Black Death, which killed 50 million Europeans in the 14th Century and the Plague of Justinian, which wiped out half of the globe in the Byzantine Empire 800 years earlier -- suggesting that they both were caused by distinct strains of the same pathogen.

Dave Wagner, a professor in the Center for Microbial Genetics and Genomics at Northern Arizona University, warned that the devastating outbreak could strike in the future. "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," he said. "If the Justinian plague could erupt in the human population, cause a massive pandemic, and then die out, it suggests it could happen again."

Microscopic samples of the plague bacteria were taken from the skeletons belonging to the two victims of the Justinian plague who were buried in Bavaria, Germany. Fragments of DNA were found in their 1,500-year-old teeth and used to recreate the bacteria's whole genetic code. Researchers then compared it with a database of hundreds of modern plague pathogens, some of which still kill thousands every year.

Published in The Lancet Infectious Diseases, the study shows the strain that caused the Justinian plague was an evolutionary "dead-end" and distinct from strains found later in the Black Death and subsequent pandemics. For instance, a third pandemic, which spread from Hong Kong across the globe, is also likely a descendent of the Black Death strain and thus much more successful than the one responsible for the Justinian Plague.

"The research is both fascinating and perplexing, it generates new questions which need to be explored," said Professor Hendrik Poinar, an ancient DNA expert. "For example why did this pandemic, which killed somewhere between 50 and 100 million people, die out?"

One possible answer is that humans evolved to deal better with the bacteria, lessening its effect as a plague. "Another possibility is that changes in the climate became less suitable for the plague bacterium to survive in the wild," Professor Wagner added. "Fortunately, we now have antibiotics that could be used to effectively treat plague, which lessens the chances of another large scale human pandemic."