Two new cases of the plague have popped up — here's why it keeps appearing in the US

This week, the New Mexico Department of Health confirmed that a 52-year-old woman and a 62-year-old woman were hospitalized with cases of the plague. Both cases were in Santa Fe county.

These aren't isolated incidents — there was another human plague case in Sante Fe county this year and four cases each in New Mexico in 2016 and 2015. There are usually a few plague infections in the US every year, mostly in the West.

References to "the plague" call to mind medieval times, with masked plague doctors wandering the streets and a "Black Death" spread by rats (or really, the fleas they carry).

RELATED: Bubonic plague through history

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Bubonic plague through history
Yersinia Pestis Bacteria Causes Bubonic Plague In Animals And Humans And Usually Is Transmitted By The Bite Of Infected Rat Fleas. Illustration Based On Light Microscope Image At 1000X. (Photo By BSIP/UIG Via Getty Images)
UNSPECIFIED - CIRCA 2003: Yersinia pestis (formerly Pasteurella pestis), the bubonic plague bacterium, seen under a microscope. (Photo by DeAgostini/Getty Images)
UNITED KINGDOM - APRIL 12: This European amulet, which features a representation of the Virgin Mary on the obverse, would have been worn as protection against catching the plague. The plague is caused by infection with Yersinia Pestis, carried by fleas that infest rodents which then bite humans. The plague is thought to have originated from the Eastern provinces of China, and travelled along the well established Silk Road through the Middle East and the Eastern Mediterranean, before reaching southern Italy in 1347 and the rest of Europe soon after. Also known as the bubonic plague and the Black Death, it killed one third of the population of Europe. Outbreaks of the plague continued to occur in Europe until the 17th century. (Photo by SSPL/Getty Images)
UNITED KINGDOM - JANUARY 21: This pomander, with a rat engraved on its side, contains six compartments and has a chain for suspension. It was probably carried as a protector against the plague. Pomanders were popular in Medieval times. They contained sweet-smelling herbs and spices and were believed to ward off infections carried by foul-smelling air. Regular outbreaks of bubonic plague, a disease transmitted from rats to people by fleas, occurred in Medieval times and continued until the 17th century. The most notable plague outbreak was the Black Death of the 14th century, which originated in China, and is estimated to have killed a third of the population of Europe. (Photo by SSPL/Getty Images)
UNITED KINGDOM - APRIL 04: A metal cautery for cauterising plague buboes (a swollen inflamed lymph node in the armpit, neck or groin). Cautery irons were heated until red-hot like branding irons, and applied to burn and seal bleeding areas, such as buboes, skin ulcers or amputation stumps. The long handle allowed the physician to keep his distance from the patient. Regular outbreaks of bubonic plague, a disease transmitted from rats to people by fleas, occurred in Medieval times and continued until the 17th century. The most notable plague outbreak was the Black Death of the 14th century, which originated in China, and is estimated to have killed a third of the population of Europe. (Photo by SSPL/Getty Images)
Early Nineteenth Century engraving of a black rat similar to that which carried the fleas that spread the bubonic plague in crowded urban areas during the Great Plague of London, an outbreak which killed some 70, 000 persons. (Photo by Time Life Pictures/Mansell/The LIFE Picture Collection/Getty Images)
Plague (19th century depiction), Bubonic plague is a zoonotic disease, circulating mainly among small rodents and their fleas. (Photo by Universal History Archive/UIG/Getty Images)
Plague (19th century depiction), Bubonic plague is a zoonotic disease, circulating mainly among small rodents and their fleas. (Photo by Universal History Archive/UIG/Getty Images)
Nineteenth Century English engraving of ghastly scene of Death Cart empyting corpses into a mass grave or Plague Pit at night during the Great Plague of London, an outbreak of bubonic plague which killed some 70, 000 persons. (Photo by Time Life Pictures/Mansell/The LIFE Picture Collection/Getty Images)
Victims of the Black Death being buried at Tournai, then part of the Netherlands, 1349. The Black Death was thought to have been an outbreak of the bubonic plague, which killed up to half the population of Europe. From the 'Chronique et Annales de Gilles le Muisit'. (Photo by Hulton Archive/Getty Images)
People praying for relief from the bubonic plague, circa 1350. Original Artwork: Designed by E Corbould, lithograph by F Howard. (Photo by Hulton Archive/Getty Images)
Late Eighteenth Century (?) English engraving of grief stricken mourners among the dead and dying in the streets of London during the Great Plague, an outbreak of bubonic plague which killed some 70, 000 persons. (Photo by Time Life Pictures/Mansell/The LIFE Picture Collection/Getty Images)
A plague hospital in Vienna during the Great Plague of Vienna, Austria, 1679. The disease, thought to be the bubonic plague, claimed around 76,000 lives. (Photo by Hulton Archive/Getty Images)
Filling a mass grave at night during the Plague of London, c 1665. Showing a group of men with torches in a churchyard, preparing to empty the contents of a covered cart into an open grave. The Plague, also known as the Black Death, was a disease caused by Yersinia Pestis, an infection carried by fleas living as parasites on rats. The Plague hit London in late 1664, having ravaged Holland the previous year, and killed around 100,000 people in and around the city. The dead were collected at night and thrown into common burial graves. (Photo by Oxford Science Archive/Print Collector/Getty Images)
An inscription on a wall of Ashwell Church, Hertfordshire, written during the epidemic of plague that swept through Europe in the middle of the 14th century known as the Black Death, May 1979. (Photo By RDImages/Epics/Getty Images)
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Plague cases now aren't too far from that. As the CDC explains, it's still a potentially fatal and serious bacterial illness. Different forms of the plague manifest in different ways. Flea bites spread bubonic or septicemic plague. Both cause fever and weakness. Bubonic plague results in painfully swollen lymph nodes; septicemic plague happens when the infection gets in the blood and causes skin and tissue to turn black and die. It can appear on its own or develop from bubonic plague.

Untreated, patients can develop pneumonic plague, the most serious form of the disease, which spreads when infected people cough droplets into the air.

The Yersinia pestis bacteria that circulate and cause plague today are remarkably similar to the bacteria researchers have identified as the cause of the Black Death. It's just that today, we live in an era of antibiotics — as long as the disease is caught early, we can treat it.

Why plague persists

It's possible that people can catch plague from direct contact with infected animals, according to the New Mexico Department of Health, but fleas are still usually the link between infected rodents and humans.

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Photo: CDC

Pets that are allowed to roam and hunt can bring infected fleas from dead rodents back into the home, putting you and your children at risk," said Paul Ettestad, public health veterinarian for the Department of Health, in a news release.

In wild rodent populations that harbor the bacteria, plague can thrive for a long time before humans come into contact with it. In the American West, where most US cases occur, there are a number of rodents — from rats to voles to prairie dogs — that are susceptible to plague, Ken Gage, a researcher focused on vector-borne disease at the Centers for Disease Control and Prevention, told NPR in 2014. In rural areas with semi-arid forests and grasslands, these animals spread the disease amongst themselves.

"What we see in the West here is the fleas will crawl up to the entrance of the burrow and wait for a host to come by," Gage told NPR. "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, which aren't the right hosts, but unfortunately those animals can be bitten by the fleas and get plague."

Still, the US only gets a handful of cases — usually between one and 17 every year. Plague is a bigger problem in places that have a harder time shutting down outbreaks. In the US, disease detectives try to find every person an infected individual came into contact with. That's harder in regions with humanitarian crises or ongoing conflicts, according to the World Health Organization.

In the 1400s, approximately 50 million people died of the Black Death. It's still one of the scarier infectious diseases out there, with a mortality rate between 30% and 60% if untreated. But fortunately, antibiotics make it possible to treat most cases now. Between 2010 and 2015, there were around 584 deaths.

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