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The plague, one of the deadliest bacterial infections in human history, caused an estimated 50 million deaths in Europe during the Middle Ages. The disease was then known as the Black Death.
Although the disease is extremely rare, it still occurs. In March, a man in New Mexico died of the plague after being hospitalized for the disease. In February, a person in Oregon was diagnosed with bubonic plague, likely after being infected by his cat.
It is transmitted by fleas that live on rodents. Symptoms usually appear within one to seven days of infection and include painful, swollen lymph nodes, called buboes, in the groin, armpit, or neck, as well as fever, chills, and cough.
The plague affects humans and other mammals.
According to the U.S. Centers for Disease Control and Prevention, people usually get the plague after being bitten by a rodent flea carrying Yersinia pestis, the bacterium that causes the disease, or by touching an infected animal.
Cats, which themselves become ill, can infect humans directly, while stronger dogs can simply bring the fleas back to their owners. Humans can also become ill by inhaling droplets from the cough of an infected person or animal.
The bacteria persists because low levels circulate among populations of certain rodents, the CDC says. These infected animals and their fleas serve as long-term reservoirs for the bacteria.
“The reason 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 the animals in the wild, it persists in nature and occasionally causes a limited number of human cases.”
Plague occurs naturally in rural areas of the western United States, particularly Arizona, California, Colorado, and New Mexico. An average of seven human cases of plague are reported to the CDC there each year. However, significantly more cases occur in parts of Africa and Asia.
According to the World Health Organization, 3,248 cases were reported worldwide between 2010 and 2015, including 584 deaths. The three most endemic countries are the Democratic Republic of Congo, Madagascar and Peru.
There are three types of plague: bubonic, septicemic and pneumonic. Pneumonic plague, which affects the lungs, carries a risk of direct human-to-human transmission. This was the case in the large outbreak in Madagascar in 2017, which resulted in 2,348 confirmed, probable and suspected cases and 202 deaths.
According to the CDC, the last urban plague epidemic in the United States occurred in Los Angeles from 1924 to 1925.
Modern antibiotics – streptomycin is the usual first-line treatment – can prevent complications and death if given soon after symptoms appear. The treatment is used for the two most common types of plague: bubonic and pneumonic.
“The reason the plague caused so much death and destruction in the Middle Ages is because we didn’t have antibiotics back then,” Barouch said of the plague in February.
“Although it can be a serious disease, it is usually easily treated with antibiotics as long as it is caught early. So now it is a very treatable disease. It should not create the fear that people had in the Middle Ages with the Black Death,” he said. “If someone develops symptoms consistent with the plague — usually the first symptoms are fever, chills and swollen lymph nodes — seek medical attention, because in the early stages the plague is easily treated with antibiotics.”
According to the WHO, the mortality rate from bubonic plague, if left untreated, is 30% to 60%, while pneumonic plague, if left untreated, is always fatal.
However, a variant of bubonic plague has been found in Madagascar that is highly resistant to streptomycin.
More than 80 percent of cases in the U.S. were bubonic plague, the most common form of infection. Untreated, bubonic plague can develop into the more serious pneumonic plague, which causes rapidly developing pneumonia after bacteria spread to the lungs.
There is a vaccine against Yersinia pestis, but it is recommended only for people at high risk, such as scientists who work directly with the bacteria, 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 study of experimental plague vaccines found that researchers are exploring different approaches to developing an effective vaccination against the plague.
Because different vaccine designs lead to different mechanisms of immunity, the authors conclude that combinations of different types can overcome the limitations of individual vaccines and effectively prevent a potential plague outbreak.
How do you protect yourself and your family?
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Important measures to prevent infestation include removing rodent nesting sites around your home, sheds, garages and recreational areas. To do this, remove brush, piles of stones, rubble and excess firewood.
Report sick or dead animals to the police or your local health officials; do not pick them 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 extra precautions. Use insect repellent containing DEET to prevent flea bites, and treat dogs and cats regularly for fleas. Don’t sleep with your pets, as this increases the risk of contracting plague. Finally, don’t let your pets hunt or roam in rodent habitats, such as prairie dog colonies.
CNN’s Jacqueline Howard and Mira Cheng contributed to this report.