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Plague’s been infecting people for thousands of years longer than any previous record shows — but it may not have always looked like the Black Death of medieval times or even earlier plagues, scientists reported Friday.
Tests on the teeth of Bronze Age skeletons shows plague’s been infecting people for at least 5,700 years, but fleas probably didn’t spread it and people may have infected one another directly.
“Our results show that plague infection was endemic in the human populations of Eurasia at least 3,000 years before any historical recordings of pandemics,” the team, led by researchers at the University of Copenhagen in Denmark, write in the journal Cell.
Endemic means it was always around —a constant threat.
But the earliest samples they got didn’t look like modern plague. It didn’t have the genetic mutations that lets the Yersinia pestis bacteria live inside the guts of fleas, for instance.
It also didn’t have the mutation that causes bubonic plague, the most horrific form, marked by blackened swellings of lymph glands called buboes.
And it didn’t have a mutation that lets it evade the human immune system so easily, causing overwhelming infections that carry people away in days.
These mutations first show up around 1686 BC, the researchers found.
The findings paint a picture of an infection that may have spread person to person. Pneumonic plague, an infection of the lungs, can spread from one person to another. It’s sometimes spread from animals to people that way, also.
Plague also could have caused a systemwide infection called septicemic plague. Untreated bubonic and pneumonic plague both can progress to septicemia and that’s what usually kills people.
Today, it can be treated with antibiotics. But there weren’t modern antibiotics 5,700 years ago, or even in the early 20th century, when plague epidemics spread around the world.
"This study changes our view of when and how plague influenced human populations and opens new avenues for studying the evolution of diseases,” said Eske Willerslev of the Center for GeoGenetics at the University of Copenhagen.
Yersinia pestis still causes plague, and it’s still occasionally infecting people. It’s infected 15 people and killed four of them this year in the United States and globally, 1,000 to 2,000 cases are reported to the World Health Organization every year.
But in the past, it wiped out entire populations. It drove big migrations of people as they fled outbreaks.
In medieval times, people didn't know plague was spread by fleas carried by rats. That wasn't discovered until 1898.
It was tracing and explaining some of these migrations in 3000 to 1500 BC that led the team to start looking for evidence of plague in earlier times.
"Perhaps people were migrating to get away from epidemics or re-colonizing new areas where epidemics had decimated the local populations. Could it be, for example, that plague was present in humans already in these prehistoric times?" asked Morten Allentoft of the University of Copenhagen.
They sequenced DNA from the teeth of 101 sets of Bronze Age remains found in Europe and Asia. They found Y. pestis DNA in seven of them and compared the mutations to modern samples of the bug to create an ancestral tree for the bacteria.