The plague is one of the most deadly infectious diseases in human history, and cases still occur today.

The plague is one of the deadliest bacterial infections in human history, known as the Black Death in medieval Europe, and killed an estimated 50 million people.

Though extremely rare, the disease still exists today: a hospitalized man in New Mexico died from the plague in March, and someone in Oregon was diagnosed with bubonic plague in February, likely contracted from a pet cat.

It is transmitted by fleas that live on rodents. Symptoms usually appear within one to seven days of infection and include painful and swollen lymph nodes in the groin, armpits and neck (buboes), fever, chills and cough.

How is the plague transmitted?

Plague affects humans and other mammals.

According to the US Centers for Disease Control and Prevention (CDC), people usually become infected with plague through the bite of rodent fleas that carry the bacteria Yersinia pestis or by handling infected animals.

Cats can infect humans directly if they get sick, while hardier dogs may just carry the fleas home to their owners, and humans can also get sick by inhaling droplets from an infected person or animal's cough.

According to the CDC, the bacteria persists because it circulates at low levels among certain rodent populations: these infected animals and their fleas act as long-term reservoirs of the bacteria.

“The reason this virus hasn't been eradicated is because it has 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 the wild and occasionally causes infection in a limited number of humans.”

Where is the plague transmitted?

Plague occurs naturally in rural areas of the western United States, especially in Arizona, California, Colorado, and New Mexico. An average of seven cases of plague are reported to the CDC each year in these areas. However, many more cases occur in parts of Africa and Asia.

According to the World Health Organization (WHO), between 2010 and 2015, 3,248 cases were reported worldwide, 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 infects the lungs, making it potentially spreadable from person to person. This was the case during the 2017 Madagascar outbreak, 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 between 1924 and 1925.

How worried should you be?

Modern antibiotics (usually streptomycin is the drug of first choice) can prevent complications and death if given soon after symptoms appear. This treatment is used for the two most common types of plague: bubonic and pneumonic.

“The reason the plague caused such widespread death and destruction in the Middle Ages was because there were no antibiotics,” Barouch said of the plague in February.

“Although it can cause serious illness, if caught early it is easily treatable with antibiotics. So plague is now a very treatable disease and you don't need to be as scared of it as people in the Middle Ages were of the Black Death,” he said. “If you experience symptoms consistent with the plague (usually early symptoms are fever, chills and swollen lymph nodes), see a doctor. In the early stages, plague is easily treatable with antibiotics.”

According to the WHO, bubonic plague has a mortality rate of 30-60 percent if left untreated, while pneumonic plague is always fatal if left untreated.

However, strains of bubonic plague highly resistant to streptomycin have been identified in Madagascar.

Bubonic plague accounts for more than 80% of cases in the United States and is the most common form of infection. If left untreated, bubonic plague can turn into the more severe pneumonic plague, in which the bacteria spreads to the lungs and causes rapidly progressive pneumonia.

Is there a vaccine for the plague?

A plague vaccine exists, but it's only recommended for high-risk people, such as scientists who work directly with the bacteria, Dr. Harish Moorjani, an infectious disease specialist at New York's Phelps Hospital, part of Northwell Health, said in February.

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

A 2019 review of plague vaccines suggests that researchers are exploring different approaches to developing an effective plague vaccination.

Because different vaccine designs induce different mechanisms of immunity, combining different types of vaccines may be able to overcome the limitations of individual vaccines and effectively prevent potential plague outbreaks, the authors conclude.

How do you protect yourself and your family?

Important steps in preventing plague include removing brush, rock piles, trash and excess firewood around the home, shed, garage and recreation areas to eliminate rodent nesting sites.

Report sick or dead animals to the police or local health authorities. Do not pick up or touch them yourself. If you must handle a sick or dead animal, wear gloves.

If you live in an endemic area, take additional precautions: Use an insect repellent containing DEET to prevent flea bites and treat your dogs and cats for fleas regularly. Don't sleep with your pets, as this increases your risk of contracting the plague. Finally, make sure your pets don't hunt or roam in rodent habitats, such as prairie dog colonies.