Plague is among the deadliest bacterial infections in human history. Cases still happen today

By Susan Scutti and Katie Hunt

(CNN) — Plague, one of the deadliest bacterial infections in human history, caused an estimated 50 million deaths in Europe during the Middle Ages when it was known as the Black Death.

While extremely rare, the disease is still around today, with a man in New Mexico dying of plague in March after being hospitalized for the disease and a person in Oregon being diagnosed with bubonic plague in February after likely being infected by their pet cat.

It’s transmitted by fleas that live on rodents. Symptoms usually appear within one to seven days after infection and include painful, swollen lymph nodes, called bubos, in the groin, armpit or neck areas as well as fever, chills and coughing.

How do you get the plague?

Plague affects humans and other mammals.

Usually, people get the plague after being bitten by a rodent flea carrying Yersinia pestis, the bacterium that causes the disease, or by handling an infected animal, according to the US Centers for Disease Control and Prevention.

Cats, which become sick themselves, can directly infect humans, while hardier dogs may simply carry the fleas back to their owners. People also can become sick by inhaling droplets from the cough of an infected person or animal.

The bacteria persists because low levels circulate among populations of certain rodents, according to the CDC. These infected animals and their fleas serve as long-term reservoirs for the bacteria.

“The reason why it hasn’t been eliminated is because there’s an animal reservoir,” Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center, said in February. “The bacteria can infect animals, and because we can’t treat all animals in the wild, it persists in nature and thus occasionally causes a limited number of human cases.”

Where can you get the plague?

Plague occurs naturally in rural areas in the western United States, particularly Arizona, California, Colorado and New Mexico. That’s where an average of seven human plague cases are reported each year to the CDC. But significantly more cases occur in parts of Africa and Asia.

From 2010 to 2015 there were 3,248 cases reported worldwide, including 584 deaths, according to the World Health Organization. It said the three most endemic countries are the Democratic Republic of the Congo, Madagascar and Peru.

There are three types of plague – bubonic, septicemic and pneumonic plague. With the pneumonic form of plague – which means it affects the lungs – there is a risk of direct transmission from human to human. That was the case in the large outbreak in Madagascar in 2017, where there were 2,348 confirmed, probable and suspected cases and 202 deaths.

The last urban plague epidemic in the United States occurred in Los Angeles from 1924 through 1925, the CDC said.

How worried should I be?

Modern antibiotics – streptomycin is the usual first-line treatment – can prevent complications and death if given promptly after symptoms appear. The treatment is used for the two most common types of plague: bubonic and pneumonic.

“The reason why it caused such widespread death and destruction in the Middle Ages is because we didn’t have antibiotics at that time,” Barouch said in February about plague.

“Although it can be a serious illness, it’s usually easily treatable with antibiotics as long as it’s caught early. So now it’s a very treatable disease. It shouldn’t create the fear that people had in the Middle Ages of the Black Death,” he said. “If anyone develops symptoms consistent with the plague — usually the initial symptoms are fever, chills and swollen lymph node — then seek medical attention, because at the early stages, the plague is easily treatable with antibiotics.”

Bubonic plague has a case-fatality ratio of 30% to 60% when left untreated, while pneumonic plague, when left untreated, is always fatal, according to WHO.

However, a strain of bubonic plague with high-level resistance to streptomycin has been seen in Madagascar.

More than 80% of US cases have been the bubonic form, which is the most common form of infection. Untreated bubonic plague can turn into the more serious pneumonic plague, which causes rapidly developing pneumonia after bacteria spread to the lungs.

Is there a vaccine for the plague?

There is a Yersinia pestis vaccine, but it is recommended only for high-risk people like scientists who work directly with the bacterium, Dr. Harish Moorjani, an infectious disease specialist at Phelps Hospital in New York, part of Northwell Health, said in February.

“Most people don’t need the vaccine,” Moorjani said.

A 2019 review of experimental plague vaccines suggests that researchers are exploring a variety of approaches to develop an effective plague inoculation.

Since different vaccine designs lead to different mechanisms of immunity, the authors conclude that combinations of different types might overcome the limitations of individual vaccines and effectively prevent a potential plague outbreak.

How do you protect yourself and your family?

Key steps for prevention of plague include eliminating nesting places for rodents around your home, sheds, garages and recreation areas by removing brush, rock piles, trash and excess firewood.

Report sick or dead animals to law enforcement or your local health officials; do not pick up or touch them yourself. If you absolutely must handle a sick or dead animal, wear gloves.

If you live in an endemic area, take added precautions. Use insect repellent that contains DEET to prevent flea bites and treat dogs and cats for fleas regularly. Do not sleep with your pets as this increases your risk of getting plague. Finally, your pets should not hunt or roam rodent habitats, such as prairie dog colonies.

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Editor’s Note: This is an updated version of a story first published in 2019.

CNN’s Jacqueline Howard and Mira Cheng contributed to this report.