When you hear the word “plague,” the Middle Ages likely come to mind.

The bacterial infection killed an estimated of the European population in the 1300s.

Now, the so-called “black death” is making a comeback of sorts.

In June, a teenager in Colorado died from a rare strain of the infection.

In early July, a 52-year-old woman and several animals were diagnosed with the illness in New Mexico.

Then there’s another recent story about fleas with the plague found on New York City rats.

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What Exactly Is the Plague?

The plague is caused by a bacterium called Yersinia pestis, which comes from fleas on rodents. People and animals can catch it when they are handling an infected animal or are bitten by an infected flea.

Dr. Camille Hamula, a microbiologist and assistant professor from the department of pathology at the Mount Sinai Hospital in New York City, explained to Healthline that infected humans with the pneumonic version of the disease can transmit it to other people via infectious droplets, like when an infected person coughs.

In the Middle Ages, Hamula said people typically caught it by handling dead, infected bodies. They also got it from rodents and fleas.

“Rats are no longer the typical rodent vectors in the U.S.,” Hamula said.

In the United States, she said it’s found in rabbits, prairie dogs, voles, mice, grounds squirrels, rock squirrels, and wood rats.

“Cases are rare but the few cases I have seen as a hospital microbiologist involved people, often children, in contact with gophers or rabbits,” she added.

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U.S. Plague History

According to the , plague came to the United States in 1900. The latest epidemic here was from 1924 to 1925 in Los Angeles.

Since then, it’s popped up mostly in rural areas — especially in northern New Mexico, northern Arizona, southern Colorado, California, southern Oregon, and western Nevada.

There were 1,006 confirmed or probable cases of humans having plague between 1900 and 2012.

More than 80 percent of those cases have been of , one of three kinds of the infection (septicemic and pneumonic are the other kinds).

Most recently, an average of seven human cases were reported each year in the United States.

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No Reason for a Plague Panic

Hamula said reports in the media should not cause alarm.

These days, anyone with the plague can be treated with antibiotics. Left untreated, though, it is highly fatal.

Hamula said that there is some evidence that the strains involved in past outbreaks in Europe were actually more infectious than those currently circulating. 

“Of course it is hard to separate this out from the effects of poor hygiene, ineffective or harmful medical care, lack of antibiotics, and misdiagnosis, all of which were common in the past,” she added. 

To prevent transmission, Hamula said people who spend time outdoors — particularly in the southwestern United States — should be aware of potential symptoms and avoid sources of the infection, such as wild gophers and rabbits.