One of the darkest chapters in medieval Europe was the Black Death, a catastrophic plague pandemic that claimed the lives of over 25 million people in just five years. Caused by the bacterium Yersinia pestis, the disease didn’t vanish with time; instead, it adapted, allowing it to survive and infect humans for centuries. A recent study published in Science has uncovered how subtle genetic changes helped the plague become less deadly but more transmissible, ensuring its persistence well beyond the height of its deadly waves.
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Researchers analysed ancient DNA from human remains across Europe and Russia, comparing plague strains from roughly a century after the first two pandemics with earlier samples. They discovered that newer strains had fewer copies of a gene called pla, known to contribute significantly to the disease’s virulence. This gene allows the bacterium to spread quickly through the host’s body by breaking down blood clots. Laboratory tests in mice confirmed that these strains were less lethal, taking longer to kill and resulting in higher survival rates.
The study also suggests that natural selection played a role in this adaptation. As both rat and human populations declined due to the plague, a less virulent form of the disease had a better chance of spreading by keeping its hosts alive longer. This slower-spreading version of the plague may have been a crucial factor in what scientists call “epidemic burnout” — a gradual decline in the intensity of outbreaks after their initial peak. Yet over time, these milder strains disappeared, and modern cases are linked to more virulent forms of Y. pestis.
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Though plague is now treatable with antibiotics and is relatively rare, it has not disappeared. Cases still arise in parts of Africa, Asia, and the Americas. This new research not only sheds light on the historical persistence of the disease but may also help us understand the mechanisms behind long-term survival of other pathogens. As with Covid-19, even after the most acute phase, a virus or bacterium can continue to evolve in the background — a reminder that infectious diseases rarely fade without a trace.
