The medieval plague known as the Black Death is making headlines this month.

In Mongolia, a couple died of bubonic plague on May 1 after reportedly hunting marmots, large rodents that can harbor the bacterium that causes the disease, and eating the animal's raw meat and kidneys – which some Mongolians believe is good for their health.

This is the same illness that killed an estimated 50 million people across three continents in the 1300s. Nowadays, the plague still crops up from time to time, although antibiotics will treat it if taken soon after exposure or the appearance of symptoms.

Left untreated, the plague causes fever, vomiting, bleeding and open, infected sores — and can kill a person within a few days.

The ethnic Kazakh couple died in Bayan-Ulgii, Mongolia's westernmost province bordering Russia and China. It is not clear what treatment they received, if any.

The incident prompted local panic. The government ordered a quarantine for six days for the region, preventing scores of tourists from leaving the area. At least one aircraft was examined by health officials in contamination suits. After no new cases appeared by Monday, the quarantine was lifted.

Every year, according to the U.S. National Center for Zoonotic Disease,at least one person in Mongolia dies from the plague, usually after coming into contact with marmots.

But they probably don't contract the disease from eating the animal's flesh, says David Markman, a researcher at Colorado State University. A person's stomach typically kills a lot of harmful bacteria before the germs are able to cause an infection, Markman says.

Yersinia pestis, the bacterium causing the plague, lives in infected animals, particularly rodents, and is usually spread by fleas. "The vast majority of human cases are a result of contracting it from a flea bite," Markman says — just as mosquitoes transmit malaria from person to person.

A Plague Primer

The plague swept Europe 700 years ago, killing a third of the population. It was called the Black Death, possibly for dark patches caused by bleeding under the skin.

It killed millions in China and Hong Kong in the late 1800s before scientists began associating the illness with rats and eliminating rodent populations.

The plague comes in three forms. If a person gets bitten by an infected flea, they'd most likely develop bubonic plague, named for the painful lumps, or "buboes," where the bacteria multiply. The bacteria can also get into the bloodstream, causing septicemic (or blood poisoning) plague, and can also spread to the lungs, causing pneumonic plague. The World Health Organization considers this variant to be one of the deadliest infectious diseases because it is highly contagious – spread by coughing — and the fatality rate is 100 percent if untreated.

Early symptoms of the plague can mimic the flu — including lethargy and swelling or stiffness in joints and lymph nodes. If someone begins exhibiting these symptoms after coming into contact with rodents or with pets in regions where the plague exists among animal populations, they should seek medical care immediately, Markman says.

Transmission Techniques

The bacterium that causes the plague will hook onto the lining of a flea's gut and stomach, growing into a film that can clog the insect's digestive passage. The next time the flea goes for a blood meal, it pukes into whatever animal it's feeding on (usually a rodent), spreading the bacteria.

Once a rodent is infected, the illness can spread to other wild animals as well as cats, dogs and people within flea-jump range.

"What we see in the West is the fleas will crawl up to the entrance of the burrow and wait for a host to come by," says Ken Gage, who studies vector-borne diseases at the Centers for Disease Control and Prevention. "If they get on another rodent that they can live on, then they've been successful. But they can also jump on humans or on dogs or coyotes or cats."

Sometimes, that new host can transport the fleas a few miles away and spread them to other animals.

Cats, which are highly susceptible to the disease, can also pass the infection to humans directly by coughing, biting and clawing.

The 21^stCentury Outlook

In modern times, the plague periodically pops up across the globe — though at minor levels compared to its heyday. Between 2010 and 2015, there were more than 3,000 cases reported, with 584 deaths.

The bacterium thrives in dry, temperate areas like the American Southwest and in North and East Africa, South and Central Asia and parts of South America.

The U.S. tends to see between one and 17 human cases a year. According to the Centers for Disease Control and Prevention, the disease likely hitched a ride to the U.S. in 1900 on flea-infested rats, which had boarded steamships in Asia. Since then, infected fleas have taken up residence on rodents including chipmunks, squirrels and prairie dogs across the southwest.

Between 2015 and 2017 in New Mexico, there were 11 cases of the plague in humans, including one death. Paul Ettestad, a public health veterinarian for the New Mexico state health department, says prairie dogs are particularly vulnerable to plague. If a whole colony gets the illness, the bacteria amplify.

"It's like putting a match to a grass prairie," he says. "Whoosh."

In places with poor access to health care, the illness can be deadly on a larger scale. That's what happened in Madagascar. The country sees between 280 and 600 infections annually. But in August 2017, health authorities began seeing an uptick in cases — particularly in pneumonic plague. After more than 200 deaths, the outbreak was contained by late November 2017. Medical teams confirmed suspected cases, treated patients quickly, advised the use of face masks to prevent infection and monitored international travel.

But it's hard to declare a permanent end to an outbreak.

The plague can persist in rodent populations, especially wild ones, for decades without affecting humans – and then can re-emerge.

Markman's research indicates that the plague bacterium can survive and multiply in microbes in soil and water. Markman hypothesizes that when ground-dwelling rodents, like marmots and prairie dogs, dig in the soil, they may encounter the bacterium, then spread it through fleas.

But he cautions that more research needs to be done, saying there are likely many reasons why the plague is still around in 2019.

Melody Schreiber (@m_scribe on Twitter) is a freelance journalist in Washington, D.C.

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