New Study Reveals How the Plague Became Less Deadly to Survive Longer

In a groundbreaking genetic study, researchers have discovered how the bacterium responsible for one of history’s deadliest diseases — the plague — evolved over centuries to become less lethal, allowing it to persist in human and animal populations far longer than previously understood.

The study, published by scientists from McMaster University in Canada and the Pasteur Institute in France, analyzed ancient DNA from victims of the plague across several centuries, from the 6th-century Justinian Plague to the 14th-century Black Death and more recent outbreaks in Africa.

The key finding centers around a gene called pla, which enables the bacterium Yersinia pestis to bypass the immune system and spread rapidly through the human body. In its most virulent historical forms, the bacterium carried multiple copies of this gene, which caused devastatingly fast and fatal infections.

However, over time, the bacterium began shedding extra copies of the pla gene, resulting in strains that were less deadly but more sustainable in the long term. These genetically altered strains allowed infected individuals — especially rodents, which serve as the natural reservoir — to survive longer, giving the bacteria more time to spread.

“This is a textbook case of evolution in action,” said the researchers. “The pathogen adapted to its hosts by becoming less virulent. The longer the host lives, the greater the chance the bacteria have to transmit to new hosts.”

The findings suggest that this evolutionary trade-off played a major role in allowing the plague to resurface repeatedly for over 500 years, particularly in Europe, while gradually decreasing its fatality rate.

Importantly, the study also highlighted the pivotal role of rodents and fleas in sustaining the disease cycle, with humans often being incidental hosts. These animal populations allowed the bacterium to continue evolving even when human outbreaks waned.

Today, while the plague still exists in parts of the world such as Madagascar and the Democratic Republic of the Congo, it is far less deadly and treatable with modern antibiotics — a result, in part, of the same genetic changes that allowed the bacterium to persist through the centuries.

The researchers emphasize that understanding this evolutionary strategy offers key insights not only into the history of pandemics but also into how future pathogens might adapt for survival — not necessarily by killing faster, but by spreading smarter.