An Australian scientist has helped explain the mystery of two of history's most deadly plagues, the Black Death and Plague of Justinian.
The plagues were caused by distinct versions of the same bug, Yersinia pestis, say scientists from around the world who studied 1500-year-old teeth from two victims of the Justinian plague.
"We discovered that the bacterium responsible for the Plague of Justinian faded out on its own," said University of Sydney professor Edward Holmes, co-lead author of a study report in the journal Lancet Infectious Disease.
The plague jumped from rats to humans and killed millions of people in the sixth century.
The scientists found the Justinian outbreak was an evolutionary dead-end and distinct from strains involved in the Black Death and other plague pandemics.
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"This study raises intriguing questions about why a pathogen that was both so successful and so deadly died out," Prof Holmes said.
"One possibility is that human populations evolved to become less susceptible."
Although the plague-causing bugs still exist in rodents in some parts of the world, there is little chance of a new pandemic, thanks largely to modern antibiotics.
However, the study could help scientists understand the dynamics of modern infectious disease.
The scientists used samples from ancient graves in the German town of Aschheim to reconstruct the oldest pathogen genome ever obtained.
They compared it with a database of Yersinia pestis genomes of more than a hundred contemporary strains.
