For centuries, the specter of the "Great Mortality" hung over the world, a recurring nightmare that reshaped civilizations, toppled empires, and left an indelible mark on the human genome. The bacterium responsible, Yersinia pestis, has long been studied as the ultimate biological predator, yet a fundamental mystery remained: how did this pathogen manage to persist in human populations for hundreds of years, appearing in devastating waves before certain lineages simply vanished?
A groundbreaking study published today in the journal Science provides a definitive look into the evolutionary engine of the plague. Conducted by an international team of researchers from McMaster University in Canada and France’s Institut Pasteur, the research identifies a specific genetic mechanism—a "volume knob" of sorts—that allowed the bacterium to fine-tune its lethality. By adjusting the copy number of a single gene, the plague could transition from a rapid, high-mortality killer into a slower, more persistent infection, a strategy that ensured its survival across generations even as it eventually led some strains toward extinction.
The study addresses the core mechanics of pandemics: how they jump into human populations, the ways they adapt to cause widespread illness, and the evolutionary trade-offs they make to persist within a host population. To understand these dynamics, the researchers looked back at the three great plague pandemics that have defined recorded history.
The first, the Plague of Justinian, erupted in the mid-500s and ravaged the Byzantine Empire and Mediterranean world for three centuries before disappearing. The second pandemic, famously known as the Black Death, emerged in the 14th century and remains the deadliest event in human history. It is estimated to have killed between 30 and 50 percent of the populations of Europe, Western Asia, and Africa. Far from a single event, the Black Death re-emerged in successive waves for more than 500 years, haunting the globe until the mid-19th century. The third pandemic began in China in 1855 and spread globally; while its impact is now mitigated by modern antibiotics, it remains a persistent public health threat in regions such as Madagascar and the Democratic Republic of Congo.
"This is one of the first research studies to directly examine changes in an ancient pathogen, one we still see today, in an attempt to understand what drives the virulence, persistence and/or eventual extinction of pandemics," explains Hendrik Poinar, the study’s co-senior author and director of the McMaster Ancient DNA Centre. Poinar, who holds the Michael G. DeGroote Chair in Genetic Anthropology, has spent decades decoding the secrets of ancient diseases, but this study marks a significant leap forward in connecting ancient genetic shifts to modern bacterial behavior.
The researchers focused their investigation on a gene known as pla. This gene is a high-copy component of the Y. pestis genome and plays a critical role in the bacterium’s ability to colonize a host. Specifically, pla acts as a key that allows the bacteria to bypass the human immune system undetected. Once inside, it facilitates the migration of the pathogen to the lymph nodes, where it multiplies and eventually spreads throughout the rest of the body, leading to the characteristic "buboes" of the bubonic plague.
By analyzing hundreds of samples from both ancient victims of the first two pandemics and modern victims of the third, the team performed an extensive genetic screening. They discovered that the copy number—the total number of pla genes present in a single bacterium—was not static. Instead, it fluctuated significantly over the course of a pandemic’s lifespan.
In the early, most explosive stages of the Black Death, the pla gene was found in high copy numbers. In this state, the disease was exceptionally virulent. It moved through the body with terrifying speed, killing its hosts quickly and with a high degree of certainty. However, the genetic analysis revealed a surprising shift: in later outbreaks of the same pandemic, the number of pla copies began to decrease.
This reduction in the gene’s presence had a profound effect on the disease’s behavior. In laboratory models using mice, the researchers observed that a lower pla copy number decreased mortality by approximately 20 percent. More importantly, it increased the duration of the infection. The hosts lived longer before succumbing to the disease, a change that, while seemingly beneficial to the host, was actually a calculated evolutionary move by the pathogen.
This discovery highlights a classic biological trade-off. A pathogen that kills its host too quickly risks "burning out" before it can be transmitted to a new victim. By reducing its virulence and allowing the host to survive longer, Y. pestis ensured it had a greater window of opportunity to spread.
The researchers noted a striking pattern: in both the Justinian Plague and the Black Death, this evolutionary reduction in pla copy numbers occurred approximately 100 years after the initial outbreaks. This suggests that the bacterium was reacting to its environment, finding a "sweet spot" of lethality that maximized its reproductive success. When the gene copy number dropped and the infected rats lived longer, they could travel further and interact with more of their kind, effectively ensuring the plague’s continued presence in the environment.
"The reduction of pla may reflect the changing size and density of rodent and human populations," says Poinar. "It’s important to remember that plague was an epidemic of rats, which were the drivers of epidemics and pandemics. Humans were accidental victims."
The role of the black rat (Rattus rattus) is central to this narrative. In the crowded cities of the medieval and early modern periods, these rats served as "amplification hosts." Because black rats are highly susceptible to Y. pestis, the pathogen required a stable and high-density rat population to thrive. If the plague killed the rats too fast, the fleas—the primary vectors that jump from rats to humans—would lose their hosts, and the cycle would break. By evolving to be slightly less lethal to the rats, the plague could maintain a steady reservoir of infection.
However, this evolutionary path eventually led to a dead end for certain strains. The study found that while one lineage of the second pandemic evolved into the ancestor of all modern-day strains, another lineage—the one that had undergone significant pla reduction in Europe—eventually went extinct by the early 19th century. This extinction likely reflected a final shift in the host-pathogen relationship, where the environment or the host populations changed so much that even the attenuated version of the bacteria could no longer survive.
To see if these historical patterns are still relevant today, the researchers turned to the vast collection of modern Y. pestis isolates housed at the Institut Pasteur. Javier Pizarro-Cerdá, co-senior author and director of the Yersinia Research Unit and the WHO Collaborating Centre for Plague at the Institut Pasteur, noted the difficulty of the task.
"Thanks to our international collaborators who monitor local epidemics of plague worldwide, we were able to find the unique bacterial samples used for this project, akin to finding of three rare needles in a haystack," Pizarro-Cerdá says.
Among the thousands of modern samples, the team identified three contemporary strains from Vietnam that exhibited the same pla depletion seen in the dying days of the ancient pandemics. This discovery provided a rare opportunity to test ancient theories on living bacteria.
Guillem Mas Fiol, a postdoctoral researcher at the Institut Pasteur and co-lead author of the study, emphasized the importance of this bridge between the past and present. "One of the most interesting aspects of our research was the possibility to explore a feature first observed in extinct plague strains, that could, for the first time, be experimentally tested in living contemporary bacterial strains," he says.
Despite the fascinating discovery of these lower-virulence strains, the researchers issued a stark reminder that the plague remains a formidable foe. While the Vietnamese samples show that the bacterium is still capable of "turning down the volume" on its lethality, the majority of the world’s current plague reservoirs do not share this trait.
"Although our research sheds light on an interesting pattern in the evolutionary history of plague, the majority of strains which continue to circulate today in Africa, South America and India are the more virulent ones," says Ravneet Sidhu, a PhD candidate at the McMaster Ancient DNA Centre and co-lead author. "These are the ones that were previously responsible for massive mortality."
The study concludes that the history of the plague is not a straight line of increasing or decreasing lethality, but rather a complex dance between a pathogen and its hosts. By understanding the genetic "switches" like the pla gene, scientists can better predict how modern pathogens might evolve as they move through human and animal populations. The persistence of the plague for over 1,500 years serves as a testament to the bacterium’s adaptability—and a warning that even the most ancient of killers can still find ways to survive in the modern world.
