What to know about plague

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Plague is a disease caused by the bacterium Yersinia pestis (Y. pestis). Plague bacteria are usually present in small mammals, such as rats, and in fleas that live on them. Infected fleas can pass plague bacteria on to different mammals that they feed on, including humans.

There are three types of plague:

• Bubonic plague: The transmission of bubonic plague often happens through flea bites. Bubonic plague bacteria attack lymph nodes, which are small glands that help the body fight infection. The plague bacteria cause the lymph nodes to become tender and swollen. These infected lymph nodes are called buboes.

• Pneumonic plague: Pneumonic plague is the most harmful form of plague. It affects the lungs and can develop in as little as 24 hours. If not treated quickly, it can be fatal. Pneumonic plague is also the only form of plague that can spread through human contact.

• Septicemic plague: Septicemic plague symptoms can occur as the first signs of plague, or they can be a result of untreated bubonic plague. A person with septicemic plague may develop black, dying skin tissue.

Plague bacteria have been around for thousands of years. Studies performed on two Bronze Age skeletons found the presence of plague bacteria in their DNA. The skeletons were approximately 3,800 years old.

Historically, the plague bacterium Y. pestis has been responsible for the Justinian plague, the Black Death, and the pandemic that broke out in the 19th century in southwest China.

The Justinian plague

The Justinian plague struck sixth-century Turkey and spread to Asia, Africa, Europe, and the Middle East between 541 C.E. and 750 C.E. In just 4 years, between 542 C.E. and 546 C.E., the Justinian plague killed approximately 100 million people in Asia, Africa, and Europe.

The Black Death

The most infamous plague, the Black Death, took place in Europe in the 14th century. At least a third of the population of Europe died between 1347 and 1352 from the plague, which is approximately 25 million people.

Over the next 400 years, outbreaks of the plague resurfaced over Europe. Two thirds of the population of Naples and Genoa died from the plague in 1656–1657. Between 1665 and 1666, London and Vienna each lost around 100,000 people to plague. Moscow had over 100,000 people die of plague in 1770–1771.

In total, the Black Death caused over 50 million deaths in Europe.

Outbreak in China

Around 1855, there was an outbreak of plague in Yunnan province in southwest China. The plague eventually spread to India, Australia, Japan, and North and South America in 1910–1920 through trade ships. By 1959, the plague outbreak killed over 15 million people.

The three forms of plague have differing symptoms in the human body. However, all forms of plague can cause:

• sudden fever
• weakness
• nausea
• vomiting
• diarrhea
• abdominal pain
• headache
• body aches

Bubonic plague

According to the Centers for Disease Control and Prevention (CDC), a person with bubonic plague will usually notice symptoms within 2–6 days after exposure to the plague bacteria.

Symptoms of bubonic plague include:

• one or more swollen, tender lymph nodes
• muscle pain
• feeling of discomfort

The buboes formed by bubonic plague are firm and painful and will often appear near the groin, armpit, or neck. If left untreated, these buboes can form into open, pus-filled sores.

Untreated bubonic plague can also develop into pneumonic or septicemic plague.

Pneumonic plague

Pneumonic plague can develop as quickly as 1 day after exposure to the bacteria.

Symptoms of pneumonic plague include:

• rapid heartbeat
• difficulty breathing
• pneumonia
• cough
• chest pain
• shortness of breath
• bloody or watery mucus

If left untreated, pneumonic plague can quickly lead to organ failure, shock, or death. However, recovery rates for pneumonic plague are high if a person receives treatment within 24 hours of symptoms developing.

Septicemic plague

Septicemic plague can develop from bubonic plague. It can also occur independently of other plagues.

Symptoms of septicemic plague can include:

• exhaustion
• muscle pain
• bleeding into skin or other organs
• black, dying tissue, particularly on fingers, toes, and nose

Septicemic plague progresses quickly and can be fatal if left untreated.

A person who has any symptoms of plague should seek medical attention immediately.

The most common cause of plague in humans is a bite from an infected flea. People who come into direct contact with infected fluids or tissues from animals with plague are also at risk of being affected.

Plague can also spread to people if they inhale droplets breathed out by a person, dog, or cat who has pneumonic plague. Pneumonic plague is the only form that can spread from person to person.

Doctors can diagnose plague by collecting a person’s blood, sputum, or lymph node tissue sample. Laboratory confirmation of plague can take up to 2 days, and so a doctor may start to treat a person with suspected plague already after taking their samples.

Doctors can treat plague using antibiotics. If a person has pneumonic plague, it may be necessary to place them in an isolation room in the hospital.

Although plague can start at any time of year, according to the CDC, the majority of cases in the U.S. occur from late spring to early fall. Additionally, plague is most common in rural areas, such as New Mexico, Arizona, and Colorado.

People living in areas with risk of plague should take care to avoid rodents — such as rats, squirrels, or chipmunks — and fleas. People should also avoid handling animal carcasses in these areas.

The CDC report that in the U.S., an average of seven cases of plague occur each year. Over 80% of plague cases in the U.S. have been bubonic plague. People aged 12–45 make up 50% of U.S. plague cases, although plague can affect people of any age.

According to the CDC, most human cases of plague between the 1990s and 2018 occurred in Africa. The CDC also state that most cases of plague were found in small towns or villages rather than bigger cities.

The World Health Organization (WHO) found that in 2017 the countries of the world with most prevalent levels of plague were the Democratic Republic of the Congo, Madagascar, and Peru.

There are several diseases that have similar symptoms to plague. They include:

• tularemia, a rare infectious disease passed on from small mammals to humans via infected ticks or flies
• cat scratch disease, also known as cat scratch fever, which a person can get through a bite, lick, or scratch from an infected cat
• Rocky Mountain spotted fever, a bacterial infection spread through a bite of an infected tick
• elephantiasis, a disease where parasitic worms infect the lymph system
• brucellosis, an infection that can occur after a person has consumed unpasteurized cheese or milk
• dengue virus disease, a viral infection spread by mosquitoes
• streptococcal lymphadenitis, an infection of the lymph nodes

Although plague is less common than it once was, there are still parts of the world where a person can get it. People living in rural areas of the U.S. should make sure to avoid contact with rodents, fleas, and animal carcasses.

Plague is usually easy to treat with antibiotics. However, it can be fatal if left untreated.

A person who notices any symptoms of plague should seek medical attention immediately.