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June 10, 2025

Anna Murray 

The Black Death plague pandemic was one of the most depressing times in medieval Europe. In just five years, the epidemic claimed the lives of at least 25 million people. It would turn out to be the most deadly epidemic in recorded human history.

The bacterium Yersinia pestis is the cause of the disease. At least 5,000 years have passed since it first appeared in human societies. The epidemic has evolved to prolong the lives of its hosts. As a result, the pathogen might continue to spread and infect people for centuries.

Scientists recently analyzed samples of ancient and modern Y. pestis to learn how the plague continued to infect people for hundreds of years after pandemic waves subsided. The results have been published in Science under the heading "Attenuation of virulence in Yersinia pestis across three plague pandemics."

Over time, the plague-causing bacterium changed to become less lethal. According to this new research released on May 29, the evolution has allowed it to continue infecting people in three different pandemics over more than a thousand years.

The first pandemic, known as the Justinian plague, began in the 500s and lasted for around 200 years during the beginning of the Middle Ages. Up to half of the population in Europe, western Asia, and Africa perished during the Black Death, which started in the middle of the 1300s and continued to spread for centuries. Parts of Africa are still experiencing the effects of the third pandemic of bubonic plague, which began in China in the 1850s.