Why we’ll never have another Black Death

Scientists disagree over how scared we should be by the mysterious death of a worker at a Russian plague lab. A young laboratory technician died earlier this month of an undisclosed illness after working on research at the Irkutsk Antiplague Research Institute of Siberia and Far East. Although officials have not released details about the cause of death, scientists agree that if it was caused by ordinary plague, the wider world has little to worry about. This bacterial illness is infamous for the explosive killing spree known as the Black Death, which wiped out between a third and half of the population of Europe between 1347 and 1353. Today, however, plague is curable and extremely unlikely to blow up into a new pandemic, experts say. That’s thanks to a combination of antibiotic development, scientific understanding and luck.

Experts who study plague are reluctant to say it’s been completely conquered. The World Health Organization (WHO) recently reported approximately 3,800 infections and 423 deaths from plague between 2019 and 2025. And scientists haven’t cracked all the mysteries behind the plague bacterium, known as Yersinia pestis—one of the biggest being how it hides for years in cryptic reservoirs only to spring out in unpredictable new waves of infection in humans or other animals.

The disease is endemic in rodents living in the Americas, Central Asia and Africa, says David Wagner, a plague expert who directs the Pathogen and Microbiome Institute at Northern Arizona University. “It’s constantly cycling between rodents via fleas, and it just does that in perpetuity in a manner we poorly understand, despite 100 years of research.”

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Studies of ancient DNA show plague-infected prehistoric hunter-gatherers in Siberia 5,500 years ago. Plague is known for causing three major pandemics: in addition to the one that started with the Black Death, which peaked between 1347 and 1351 and was followed by outbreaks into the 18th century, there was the plague of Justinian from C.E. 541 to 549 and a third pandemic that started in the 1800s and spread to every inhabited continent by 1900. That last pandemic was accelerated by a boom in rail and steamship travel but was eventually quashed by advances in science.

In 1894 French-Swiss bacteriologist Alexandre Yersin isolated the plague bacterium and connected rat infestations to the disease. Rat eradication campaigns followed around the world, drastically slowing the spread of plague, Wagner says. Though it was really the fleas that spread the disease to people, rat control worked because fleas are very specific about hosts. Different animal species have their own flea species, so rats and plague-carrying fleas traveled together.

Quarantines also helped, Wagner adds, because doctors started to recognize that the bacteria could sometimes spread from person to person through respiratory droplets. In the 1940s, when antibiotics had become widely available, doctors started curing plague cases with the drugs. Several vaccines have been developed over the years and deployed in severe outbreaks, in which they offered partial protection. No vaccine has been deemed effective enough to gain approval from the WHO or the U.S. Centers for Disease Control and Prevention, but labs in Russia and the U.S. continue to work on them.

Today most plague cases occur in Madagascar. Because food can be scarce there and people fear theft, individuals will store rice in their dirt-floor homes, Wagner says, allowing close contact with the rats that infest the island.

Though fleas are picky about their hosts, plague-carrying fleas in the U.S. are commonly found on prairie dogs, thanks to the animals having some functional similarities to rats. The plague bacterium now infects all five known species of prairie dogs and is one of the factors that contributed to a population decline in these social, burrowing rodents of about 98 percent from their historical levels. Because prairie dogs are a food source for other animals, including the critically endangered black-footed ferret, plague has been wreaking ecological havoc.

Even now it’s not clear where Y. pestis hides out between detectable outbreaks, Wagner says, and it’s surprising that it can be transmitted both by respiratory droplets and through fleas. But important clues about how the sickness operates are emerging from the latest studies of bacterial DNA in ancient bones and teeth.

The plague bacterium appears to have emerged as a human pathogen more than 5,700 years ago, evolving from a soil bacterium called Yersinia pseudotuberculosis, says Hendrick Poinar, a biologist at McMaster University, who works with ancient DNA. For the next 3,000 years, it infected Late Stone Age hunter-gathers and Bronze Age herders. Scientists suspect it was transmitted from human to human via respiratory droplets or via contaminated food because it hadn’t yet acquired the genes needed to travel through fleas.

Despite the versatility of Y. pestis, we’re lucky it can’t spread between humans unless an infected person is very sick and the contact is very close. Because of this limitation, even the emergence of an antibiotic-resistant strain won’t necessarily spark a new pandemic, Wagner says, noting that such strains have already shown up in Madagascar.

Though most scientists aren’t too alarmed about the situation in Russia, some are speculating that the death was caused by some highly genetically modified version of the plague bacterium or a different germ entirely. There’s no evidence for this, though the lack of transparency from Russia is unnerving. Regardless of the cause, the death of the worker is tragic. But if it was caused by the plague we know, the rest of us need not worry.