The New Mexico Department of Health said this week that two women were found to have plague, bringing the total number of people this year in the state known to have the disease to three.

All three patients — a 63-year-old man and two women, ages 52 and 62 — were treated at hospitals in the Santa Fe area and released after a few days, said Paul Rhien, a health department spokesman.

Health officials in New Mexico have more experience with plague than many might expect: Every year for the last few years, a handful of people in New Mexico have come down with plague. One person has died.

While the word “plague” may conjure images of medieval cities laid to waste by the Black Death, the disease is still a part of the modern world. It is much less common than it once was, but it is no less serious.

What Is Plague?

Plague is caused by the bacterium Yersinia pestis, which humans get when they are bitten by rodent-riding fleas. It decimated European cities during the Middle Ages, killing tens of millions of people, but today is found mostly in rural areas.

There are three main types of plague in humans, according to the Centers for Disease Control and Prevention: bubonic plague, pneumonic plague and septicemic plague. All three share general symptoms — like fever, weakness and chills — but each subtype carries its own fearsome markers.

Pneumonic plague causes a rapid and severe form of pneumonia that can lead to respiratory failure and shock. It is the only type that can be spread person-to-person through the air if someone inhales infected water droplets.

Septicemic plague, which attacks a person’s blood cells, can cause skin or other tissue to turn black and die, especially on the extremities, like hands and feet. It is caused by either an infected flea bite or by handling an infected animal.

Bubonic plague is the best-known and common form of the disease. It is marked by the sudden appearance of bulbously swollen and painful lymph nodes (called buboes) in the groin or armpits.

How deadly is Plague?

It can be very deadly. Fifty to 60 percent of the cases of bubonic plague are fatal if they are not treated quickly, according to the World Health Organization.

Paul Ettestad, the public health veterinarian for New Mexico, said plague can be treated with antibiotics like gentamicin and doxycycline, but it is important to catch it fast.

Pneumonic and septicemic plague can be more serious. The World Health Organization described them as “invariably fatal,” but there are some people who have survived these forms of the disease.

In 2002, a married couple from New Mexico contracted plague at home and developed symptoms while they were on vacation in New York. One of the patients, John Tull, developed septicemic plague.

Mr. Tull’s kidneys nearly failed, and tissue in his feet and hands turned black and began to die. He was placed in a three-month medically induced coma and doctors amputated both his legs below the knee, but he survived.

How common is plague?

Plague is a lot less common now than it was in centuries past, when millions died in repeated plague epidemics. From 2000 through 2009, there were 21,725 reported cases of plague worldwide, according to the American Society for Tropical Medicine and Hygiene. Of those, 1,612 were fatal.

Most cases of plague diagnosed since the 1990s have been in Africa, particularly Congo and Madagascar, although outbreaks have also happened in Asia and North and South America.

The American Society for Tropical Medicine and Hygiene said 56 plague cases were found in the United States — seven of them fatal — from 2000 through 2009, the last year for which figures were available.

Why does it keep happening in New Mexico?

Plague arrived in the United States around 1900 on ships from China and soon jumped from fleas on urban rodents to fleas on rural rodents, Mr. Ettestad said.

It is now “entrenched” in large swaths of the western United States, with most cases occurring in New Mexico, Colorado, Arizona, California, Oregon and Nevada, according to the C.D.C.

Plague in New Mexico has been especially persistent, Mr. Ettestad said. The state health department said it was found in four people in 2015, with one death. Four more people were found to have it in 2016; all were successfully treated.

Mr. Ettestad said there were environmental reasons that plague kept popping up in New Mexico. The area is home to vegetation like pinyon and juniper trees, which, he said, support “a wide diversity of rodents and fleas.”

That means that once plague has decimated one rodent species — say, the prairie dog — there are lots of other rodent species nearby it can jump to, like the rock squirrel.

“A lot of people have rock squirrels in their yard, and when they die, their fleas are very good at biting people,” Mr. Ettestad said. “We have had a number of people who got plague after they were bitten by a flea that their dog or cat brought in the house.”

What should I do if I think I have plague?

Medical authorities are unanimous on this: If you live or have recently returned from any area where plague is found (like New Mexico) and you develop symptoms of the disease, then you should immediately go to a doctor or hospital.