A strain of bubonic plague as deadly as the one that caused the Black Death could make a reappearance on Earth, scientists have warned.
The evidence comes from research into one of the most devastating pandemics in human history, the Plague of Justinian, which killed half the world's population.
Scientists who isolated genetic traces of the plague from the teeth of two 1,500-year-old victims found it was caused by a distinctly different bacterial strain from that responsible for the later Black Death.
The new research published online in the journal The Lancet Infectious Diseases shows that the two infectious organisms were distinct strains. While the Justinian strain vanished from the Earth, the Black Death strain gave rise to another pandemic in the late 1800s.
The discovery suggests that a new deadly strain of plague could strike again without warning.
The Plague of Justinian struck in the 6th century and is estimated to have killed between 30 million and 50 million people as it spread across Asia, North Africa, the Middle East and Europe - virtually half the world's population at the time.
Some 800 years later, the Black Death wiped out 50 million Europeans between 1347 and 1351.
The scientists managed to recover tiny DNA fragments of the bacterium from the teeth of two Justinian plague victims buried in Bavaria, Germany.