What a Pest

For a microscopic organism, Yersinia pestis has made an outsize mark on human history. It has felled some 200 million human beings since it first evolved, in addition to provoking political, economic, social, and cultural upheavals. This toll of death and devastation has earned the disease resulting from this bacterium the right to be called, simply, "The Plague." And though it certainly has a long history, this tiny killer also has a bright future.

Plague has never really disappeared, but it suddenly seems poised for a comeback. Indeed, world health officials have quietly recategorized plague as a "re-emerging" disease in recent years, and it now infects 2,000 people annually, killing 200. The Chinese government even quarantined an entire town this summer after an outbreak of pneumonic plague, which eventually killed three and infected nine more. And experts fear the next stage of the disease will be especially dangerous, fueled by age-old phenomena, such as humans trying to use plague to wage war on their enemies, as well as new ones, such as climate change.

Welcome to the plague years, the next generation. For most people, plague automatically means the Black Death, which began in the 14th century and killed a quarter to a third of Europe’s population, roughly 15 million to 25 million people. This is the best-known plague pandemic, but it wasn’t the first. That honor goes to a sixth-century outbreak that originated in northern Africa and took out as many as 100 million people. Nor was the Black Death the last major pandemic. Plague spread through China and India during the 19th century, killing some 12 million people, and then spread to the United States in 1900, causing an epidemic in San Francisco.

Between major pandemics, the plague never completely disappeared. It never does: It merely retreats until conditions favor another outbreak. Plague "would be virtually impossible to eradicate," says Ken Gage, chief of the flea-borne diseases branch of the U.S. Centers for Disease Control and Prevention. "The reservoir is rodents, and it’s very widespread." Indeed, Y. pestis is endemic in many populations of rats, mice, squirrels, marmots, gerbils, and other rodents. Fleas that feed on these infected animals can pass the disease along to humans. The unlucky recipients usually come down with flu-like symptoms, and death is often a torturous ordeal. All three forms of the disease — bubonic plague, pneumonic plague, and septicemic plague — can be easily cured with antibiotics, assuming it’s caught early. When it isn’t, the prognosis is poor, with a mortality rate of 50 to 100 percent.

Today, plague is endemic among the rodents of the American Southwest. Isolated outbreaks also occur regularly in East and Southern Africa, Vietnam, Burma, China, Mongolia, Russia, and Central Asia. What’s more, there are already troubling signs that the disease is evolving into even more dangerous forms: Scientists recently discovered a drug-resistant strain of the plague in Madagascar.

A perfect storm of factors — including population increases, urbanization, deforestation, changing land use, migration, and the growing ease of travel and trade — are all playing a role in plague’s re-emergence. So too, scientists think, is climate change. During the Black Death, the climate was warmer and wetter than usual, which "was very favorable for the plague system," says Nils Stenseth, a biologist and plague expert at the University of Oslo. Stenseth’s research in modern Kazakhstan also revealed that for every 1 degree Celsius increase in the spring temperature, there’s a whopping 60 percent increase in the incidence of plague among the gerbils that play host to Y. pestis. Earlier springs and increased precipitation would benefit the plague pathogen, which is particularly troubling because climate change is expected to have exactly these effects in many parts of the globe.

At the same time, climate change could dramatically alter the global distribution of plague, meaning that its past won’t necessarily be the best guide to its future. Some regions could become too hot and dry for the fleas that transmit the disease, causing plague to fade away there while popping up in newly hospitable areas. In the United States, for instance, plague cases are currently centered in New Mexico, but climate change is expected to expand the range of the disease all the way up to Wyoming and Idaho.

And there are even more direct threats than climate change that could resurrect plague in a big way — like bioterrorism. Plague warfare is almost as old as the plague itself. In the 14th century, the Tatars catapulted plague-ridden bodies into a town controlled by their Italian enemies. Russians employed a similar tactic when battling the Swedes four centuries later. And during World War II, the Japanese reportedly dropped plague-infected fleas from airplanes while flying over Chinese territory. Modern bioterrorists would probably be even more sophisticated, encapsulating the bacteria in droplets of liquid and spraying them into the air. (In fact, during the Cold War both the United States and the Soviet Union developed techniques to aerosolize plague.) In 1970, the WHO calculated that releasing 50 kg of plague in aerosol form over a city of 5 million could lead to 150,000 plague infections and 36,000 deaths.

And that’s without genetic alterations to the pathogen, giving it resistance to antibiotics or making it live longer outside the host so it could spread through the air, which is exactly what some fear is coming next. For obvious reasons, information on whether particular terrorist groups or rogue states are actively working to weaponize plague is sparse to nonexistent. (But in 1995, a white supremacist in Ohio was arrested after illegally obtaining the plague bacteria by mail.) Nevertheless, experts remain convinced that plague is an appealing bioterrorist agent and caution that we should be prepared for its possible use.

Still, plague is unlikely to do as much damage in the modern world as it once did. The spread of plague can be interrupted pretty easily through simple steps such as wearing face masks and avoiding sick people. No large-scale quarantine is necessary.

That’s a good thing, because plague will always be a looming threat. "If we’re going to eradicate plague, we have to get rid of all the rodents," Stenseth says. "That’s an impossible job. Plague will never, ever go away."