The plagues deadliest outbreak was in the 1300s but is it gone for good? In this one-minute video, our Ask Smithsonian Host, Eric Schulze, explains what causes the plague and what we are doing to make sure it doesnt happen again.

Ask Smithsonian: Is the World Due for Another Massive Plague Outbreak?

It is highly unlikely, experts say, but a plague-based bioterror assault is another matter

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The specter of another plague like the one that affected Eurasia in the middle of the 14th century has hung like a ghastly pallor over the intervening centuries. But the likelihood of a naturally occurring outbreak happening on that massive a scale is extremely unlikely in modern times.

The Black Death of the 1300s has been noted as one of the worst pandemics in recorded history, killing 25 to 30 percent of the inhabitants of Europe, North Africa and the Near East and possibly affecting as many in China, India and the Far East as well. Some 50 million people died.

Unsanitary living conditions and a complete ignorance of what caused the disease or how it was transmitted fueled the spread of the plague across continents. As the world has developed, plague has become less of a threat. The World Health Organization estimates that in 2013, for instance, there were just 783 cases of plague across the globe, resulting in 126 deaths.

Modern sanitation and scientific and medical advances have had a big impact. Humans in the 21st century are also a lot different than the ones who walked the Earth 700 years ago; they’re better-nourished and have stronger immunity, says Amesh Adalja, a senior associate at the University of Pittsburgh Medical Center’s Center for Health Security.  “It’s the effect of civilization overall that has made plague less likely,” says Adalja.

Adalja says he “highly doubts that a massive plague outbreak” could occur again—at least on its own. But an epidemic fueled by a plague-based bioterror weapon is another issue.

The Black Death was due to the naturally occurring bubonic form of plague. Rats infested with fleas carrying the Yersinia pestis bacterium were sharing close quarters with humans. Now, as it was then, people bitten by infected fleas will develop flu-like symptoms within days, with a sudden onset of fever, chills, aches, nausea and vomiting.

If left to its own devices, the bacteria quickly replicate in lymph nodes; the nodes spread the agent throughout the bloodstream, leading to hemorrhaging and eventually septicemia (blood poisoning) and death. Often, if the victim lives long enough, the buboes burst, becoming open sores that ooze bacteria. The infection tends to kill within days or a week. Even after death, cadavers can still be infectious.

Fourteenth century humans didn’t understand infectivity, but when the Black Death began ravaging the Crimean peninsula, the Tartars—who had been engaged in a multi-year war in the area—began catapulting the suppurating bodies of plague victims over the walls of Kaffa, today’s Feodosia in Ukraine. The plague likely spread inside the city as people tried to move the mangled, odiferous cadavers. Many researchers cite the attack as the first known episode of biowarfare, says Adalja.

Meanwhile, the bubonic plague spread throughout Eurasia along shipping routes, thanks to rats that stole aboard the vessels. Eventually that outbreak died down, but there have been others throughout the centuries. No one truly understood what caused plague or how it spread until Swiss scientist Alexandre Yersin discovered a bacterium—isolated from buboes­­—while investigating an outbreak in China in 1894. The microbe was later named Yersinia in his honor.

Meanwhile, the plague continues to surface periodically, particularly in Africa, Asia and South America.

The disease did not get to the United States until 1900, arriving in San Francisco, via China, carried by infected rats riding on steamships. More than 100 people died within the decade. The plague then spread south to Los Angeles causing a short epidemic in 1924. Harbored by rats and other small animals, the plague made its way out to the desert east of Los Angeles and then kept going.

Most of the American cases—anywhere from one to 17 a year—occur in northern New Mexico, northern Arizona, southern Colorado, California, southern Oregon and western Nevada.

This year, a Colorado teenager died from the bubonic plague in June, and an adult from the state died in August. A Michigan woman was diagnosed in September, after traveling to Colorado.

Plague isn’t always transmitted by fleas. It can also be spread when someone expels the bacteria through coughing or sneezing. The aerosolized droplets can be inhaled, leading to pneumonic plague.

In May, the U.S. Centers for Disease Control and Prevention reported on a 2014 outbreak in Colorado, in which a dog got plague and then transmitted it to its owner by coughing. Two veterinary employees also were infected by the dog, and another worker may have been infected by the owner. All had the pneumonic form, and all survived, except the dog.

Left untreated, the plague kills 90 percent of those infected. But it can be cured with common antibiotics, and with treatment, only about 16 percent of those infected with any plague will die.

A vaccine, however, has been more elusive. Researchers have tried to improve on a vaccine that was used beginning in the late 1800s to no avail. Even though the U.S. has few cases, a vaccine would be useful in developing countries, says Adalja. And, he adds, “plague is a national security threat, and that has fostered a lot of investment in developing countermeasures.”

The Japanese Imperial Army developed and used plague as a weapon of war, launching at least one attack against the Chinese in 1940, and was preparing to use it in the U.S.

The former Soviet Union had a large bioweapons program, including plague-based weapons, as did Iraq, and both North Korea and Syria are rumored to have bioweapons, Adalja says.

The U.S. had an offensive bioweapons program until 1969, he says. Now, “we have some of these pathogens to develop defensive measures,” including vaccines and treatments, Adalja adds.

The plague—especially the pneumonic form—could be an effective weapon, in part because it might not be immediately diagnosed. And that’s probably about the only way a new Black Death pandemic could occur.

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About Alicia Ault
Alicia Ault

Alicia Ault is a Washington, DC-based journalist whose work has appeared in publications including the New York Times, the Washington Post and Wired. When not chasing down a story from our nation's capital, she takes in the food, music and culture of southwest Louisiana from the peaceful perch of her part-time New Orleans home.

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