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Black Death 'Could Strike Again' Warning
A strain of bubonic plague as deadly as the one that caused the Black Death could strike again without warning, scientists have warned
Researchers studied the Plague of Justinian, which killed half the world's population in the 6th century and found it was caused by a different strain to the Black Death in the 14th century.
The Justinian strain vanished from the Earth, but the Black Death gave rise to another pandemic in the late 1800s.
The study - which used genetic traces from the teeth of two 1,500-year-old victims - suggests that a new strain of plague could reappear without warning.
Both diseases were caused by the Yersinia Pestis bug, which can be spread by fleas from rats to humans.
Northern Arizona University's Dr Dave Wagner said: "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.
"If the Justinian plague could erupt in the human population, cause a massive pandemic, and then die out, it suggests it could happen again.
"Fortunately we now have antibiotics that could be used to effectively treat plague, which lessens the chances of another large-scale human pandemic."
The Plague of Justinian is thought to have killed between 30 million and 50 million people as it swept through 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 research, published by journal The Lancet Infectious Diseases, revealed that the strain responsible for the Justinian plague was an evolutionary "dead end" - doing its damage and disappearing long before the Black Death struck.
The 19th century pandemic which spread across the globe from Hong Kong was a likely descendent of the more successful Black Death strain, said the scientists.
There was no molecular evidence of a link with earlier smaller epidemics such as the Plague of Athens in 430 BC and the Antonine Plague between the years 165 and 180.
These outbreaks could also represent separate, independent emergences of Y pestis strains capable of infecting humans, it is believed.
The report's Australian co-author Dr Edward Holmes said: "This study raises intriguing questions about why a pathogen that was both so successful and so deadly died out.
"One testable possibility is that human populations evolved to become less susceptible."