The Black Death did not end in 1353. It opened a pandemic that returned across Europe for more than four centuries, and a new genetic study shows the bacterium behind it kept changing along the way. Around 1450 to 1500, the plague bacterium Yersinia pestis diversified into three major branches that may have established new reservoirs in wild rodents, according to research published this month in the Proceedings of the National Academy of Sciences.

The researchers speculate that climate played a role. One possible factor, they say, is the Great Renaissance Drought, a period of severe dryness in Europe. Modern studies have shown that climate strongly influences plague among wild rodents, which carry the disease between human outbreaks.

The finding reaches beyond history. Plague still circulates in wild rodents today, including in parts of the western United States, and understanding how the bacterium settled into animal populations centuries ago could help scientists watch for changes in modern disease hot spots.

Reading Plague DNA from Medieval Graves

The team, led by researchers at the University of Tartu in Estonia, reconstructed 26 Yersinia pestis genomes from human remains at 11 archaeological sites in Estonia, Russia, England, the Netherlands and Switzerland, according to the University of Stirling. The samples span the 14th through 18th centuries, the era historians call the Second Plague Pandemic.

The biggest challenge was time, as ScienceDaily's summary of the study explains. Genomes from modern outbreaks come with exact collection dates, but ancient burials are usually dated by radiocarbon methods whose ranges can span more than a century. That makes it hard to link a genome to a specific outbreak.

To tighten those windows, the researchers developed a method they call Phylogenetically Informed Radiocarbon Modeling. Because the bacterium accumulates small genetic changes over time, a genome's position on the family tree carries information about when it lived, and the team combined that with radiocarbon dates. "We were able to improve dating intervals for many samples," historian and corresponding author Philip Slavin of the University of Stirling said in the research announcement, adding that this let the team connect samples to outbreaks recorded by local chroniclers.

A Pandemic That Kept Branching

The results show plague did not simply linger in one place after the Black Death. Instead, it resurfaced repeatedly in different parts of Europe over several centuries. "We found evidence for repeated introductions of plague into Estonia starting already in the late 14th century," said senior author Kristiina Tambets, who added that the team identified several previously unknown genetic lineages in both urban and rural settings.

The late-15th-century branching stands out. During that window, the bacterium split into three major lineages, and the authors say some of them may have moved into wild rodent populations and persisted there. That would help explain how plague kept returning to European towns long after the initial wave.

Human conflict also shaped the pandemic. The new genomes add evidence for outbreaks associated with the Thirty Years' War (1618 to 1648) and the Great Northern War (about 1700 to 1721), and the researchers say plague frequently spread along the same routes used by armies, refugees and traders.

The Evidence Check on the Drought Link

It is important to separate what the study shows from what it suggests. The genetic diversification around 1450 to 1500 comes from the genome data. The drought connection is a hypothesis, which the research announcement describes as speculation, and the study does not demonstrate that drought caused the branching.

This is a peer-reviewed genomic analysis based on ancient DNA. Its limits include uneven sampling, with most genomes from northern and western Europe, and dating that, while improved, still carries uncertainty. The identity of any 15th-century wild rodent reservoirs also remains unresolved, because ancient animal hosts rarely leave recoverable plague DNA.

Why would drought matter at all? Research on modern plague systems has found that climate conditions affect rodent populations and the fleas that carry the bacterium, which in turn affects how much plague circulates in wildlife. A major regional drought could plausibly have reshaped those populations, but that link has not been tested directly for the Renaissance period.

Lessons for Wildlife Disease Surveillance Today

Plague is still present in the United States, though it is rare. The Centers for Disease Control and Prevention reports an average of seven human cases a year in recent decades, most acquired in northern New Mexico, northern Arizona and southern Colorado, or in California, southern Oregon and far western Nevada. Plague reached the U.S. by ship in 1900, then spread from urban rats into rural rodents and became entrenched across much of the West.

That history mirrors what the new study describes in medieval Europe: a disease moving from human outbreaks into wild animal populations, where it can persist quietly for decades. Understanding how plague established itself, persisted, and eventually disappeared from Europe could inform modern surveillance of active plague areas.

For residents and visitors in the rural West, the practical steps are simple. Avoid handling sick or dead rodents, keep pets out of prairie dog colonies, use flea control on dogs and cats, and use insect repellent in areas with known plague activity. Seek medical care promptly for sudden fever, chills, and painful swollen lymph nodes after time outdoors in these areas. Plague is treatable with antibiotics when caught early.

The next step for researchers is more samples, especially from southern and eastern Europe and from ancient rodent remains that could identify reservoirs directly. The bottom line: the Black Death was the opening chapter of a much longer story, and the bacterium's ability to settle into wildlife is a lesson that still applies to disease monitoring today.

What Readers Want to Know

What did the new plague study find?

Researchers reconstructed 26 plague genomes from 11 European sites and found that the bacterium diversified into three major branches around 1450 to 1500, some of which may have entered wild rodent populations.

Did drought cause plague to spread?

Not proven. The researchers suggest the Great Renaissance Drought may have played a role, but the study does not show that drought caused the diversification.

How did scientists date the genomes?

They combined radiocarbon dates with each genome's position on the bacterium's family tree, narrowing the time ranges and linking samples to recorded outbreaks.

How long did the second plague pandemic last?

It began with the Black Death of 1347 to 1353 and recurred in waves across Europe for more than four centuries, into the 18th century.

Does plague still exist in the United States?

Yes. The CDC reports an average of seven human cases a year, mostly in rural areas of the West, where the bacterium circulates among wild rodents and their fleas.

How can people protect themselves?

Avoid contact with sick or dead rodents, keep pets out of rodent burrows, use flea control and insect repellent, and seek prompt medical care for sudden fever and swollen lymph nodes.