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This summary covers The Economist’s June 20th, 2026 Science & technology article listed as Ancient plagues and published under the headline Biology's plague journal.

Plague left more than mass graves and historical fear. It also left genetic evidence inside human teeth. The article explains how scientists are using that ancient DNA to reconstruct the disease’s evolution, identify outbreaks that written history never recorded and challenge the idea that plague needed crowded cities to become catastrophic.

A History Written in Teeth

The modern investigation began with a breakthrough in 2011. Researchers recovered DNA from people buried in London’s East Smithfield cemetery and found genetic material from Yersinia pestis, the bacterium that causes plague. The discovery confirmed that the Black Death was indeed a plague outbreak, turning a long-standing historical conclusion into a genetic fact.

Similar work has since shown that the Plague of Justinian, which struck the Byzantine empire in the sixth century, was also caused by Y. pestis. Yet the strain behind that pandemic was not the direct ancestor of the one that devastated Europe roughly eight centuries later. Hundreds of ancient bacterial genomes have now helped researchers trace different plague lineages, including the Black Death’s likely origin near Lake Issyk-Kul in modern Kyrgyzstan.

Teeth make this research possible because enamel protects DNA long after other tissues decay. The pulp inside a tooth was once connected to the bloodstream, so it can preserve traces of pathogens that circulated during a person’s life. In effect, each tooth can become a biological archive containing both human and microbial history.

Plague Before the City

The newest evidence pushes that history much further back. A team led by geneticist Eske Willerslev found signs of two deadly plague outbreaks among hunter-gatherers living near Lake Baikal in Siberia about 5,050 and 5,520 years ago. Nearly 40% of the people buried in four Late Neolithic cemeteries carried the bacterium. Radiocarbon dates, family relationships and shared graves suggest that the victims died in concentrated outbreaks rather than isolated infections.

Children and teenagers were especially prominent among the dead. The ancient bacterium carried genes for a toxin resembling one found in a related modern pathogen. That toxin can trigger a dangerous immune overreaction, particularly in children, which may help explain the age pattern in the graves. The evidence supports the view that prehistoric plague could be highly lethal, not merely a mild ancestor of later pandemics.

This finding also weakens a familiar theory about what made plague explosive. Medieval cities offered dense populations, abundant rats and flea-infested living conditions. The Lake Baikal victims, however, lived in small, mobile communities long before urbanisation. Their strain also lacked a mutation that helps modern plague bacteria survive inside fleas. Transmission through bodily fluids or respiratory droplets may have been enough, or another insect carried the bacterium through trade in animal skins. Similar strains later appeared thousands of kilometres away in Latvia and Sweden, suggesting either a vast rodent reservoir or long-distance human movement.

From Archaeology to Preparedness

Plague is now rare and treatable, but the research matters beyond solving ancient mysteries. By comparing bacterial genomes across thousands of years, scientists can see which mutations helped pathogens spread, which lineages disappeared and which survived. That record can guide disease surveillance and may eventually contribute to vaccine development.

The larger lesson is that pandemics do not obey a simple progression from sparse settlements to crowded cities. Dangerous pathogens were adapting to humans and moving across continents well before written history. Ancient DNA gives public-health researchers a much longer timeline from which to learn - and a clearer view of how unexpectedly disease can emerge.