Bubonic plague is an infectious disease that causes swollen, painful lymph nodes. It caused the deaths of more than 25 million people in the 14th century. The disease still occurs in many parts of the world.

A bacterium called Yersinia pestis causes plague. These bacteria can be present in small mammals and the fleas that feed on their blood.

This article explores bubonic plague in more detail, including its history, symptoms, and treatment.

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The bubonic plague is one of the three forms of plague. It is the most common form of the disease. Its name refers to the “buboes,” or swollen lymph nodes, that develop due to the disease.

Non-human animals can transmit the bacteria that cause bubonic plague to people. The disease is uncommon in modern times due to advances in living conditions and antibiotics. However, bubonic plague still exists in many parts of the world.

The Centers for Disease Control and Prevention (CDC) states that there is an average of seven human cases of bubonic plague in the United States per year. The diagnoses generally occur in rural and semi-rural areas.

The bacterium that causes bubonic plague has existed for thousands of years.

In 1347, ships carrying rats with the plague arrived in Sicily. From there, the plague spread quickly across Europe. Over 25 million people died between 1347 and 1352 as a result. This was at least a third of the population of Europe at the time.

Outbreaks of bubonic plague continued to resurface over the next 400 years. Two-thirds of the population of Genoa and Naples died due to the disease between 1656 and 1657.

Between 1665 and 1666, around 100,000 people in London died due to the disease. Reports indicate that 100,000 people also died in Vienna in 1679 because of bubonic plague. Additionally, 100,000 people died in Russia between 1770 and 1771 because of this plague.

The World Health Organization (WHO) notes that in total, bubonic plague was responsible for the deaths of more than 50 million people in Europe. In the 14th century, people referred to the disease as the “Black Death.”

Bubonic plague still exists.

The WHO states that plague is present in all continents apart from Oceania. The organization also notes that since 1990, most human plague cases have occurred in Africa.

The three countries where plague occurs most often are:

  • Democratic Republic of the Congo
  • Madagascar
  • Peru

Madagascar has reported cases of bubonic plague almost every year. These cases typically occur during the epidemic season, which is between September and April.

The CDC notes that most recent cases of plague in the United States have occurred in people living in small towns, villages, or agricultural areas.

Additionally, the CDC highlights that most plague cases affecting people in the United States occur in the following regions:

  • Northern New Mexico
  • Northern Arizona
  • Southern Colorado
  • California
  • Southern Oregon
  • Far Western Nevada

The CDC also states that more than 80% of plague diagnoses in the United States have been bubonic plague.

Generally, a person who has bubonic plague will develop symptoms 2–8 days after infection.

Once bubonic plague bacteria enter a person’s body, they travel via the lymphatic system to the closest lymph node. The bacteria then replicate inside the lymph node, causing it to swell and feel tender and painful.

In the advanced stages of bubonic plague, a person’s buboes can become pus-filled open sores.

Other symptoms of the bubonic plague include:

The CDC notes that the most common cause of bubonic plague is a fleabite. This is because plague bacteria often transmit through fleas after they feed on blood from small mammals, such as ground squirrels, prairie dogs, and chipmunks, with the disease. The bacteria travel from flea mouthparts into a new host when the flea bites them.

A person may also develop plague if they have direct contact with fluid or tissue from an animal with the disease or one that has died from it.

If a healthcare professional suspects a person has bubonic plague, they will request samples of the person’s:

Those samples are then sent to a laboratory for testing. Laboratory technicians can provide preliminary test results for these samples in less than 2 hours, with a confirmation after 1–2 days.

Without treatment, plague can be fatal. The CDC notes that a person’s chances of recovery from plague are better the sooner they receive treatment.

A healthcare professional will start treatment for suspected plague as soon as they take a person’s samples. Treating bubonic plague involves a course of antibiotics. Antibiotics are an effective treatment if a person is diagnosed in time.

The CDC notes that between 1990 and 2010, the mortality rate for plague in the United States was 11%. This rate is lower for bubonic plague than for the other two forms of the disease.

However, the mortality rate for bubonic plague may be higher in other countries. The CDC states that it can be difficult to assess the mortality rate in people living in low or middle income countries as few cases are reliably diagnosed and reported.

The WHO notes that the case-fatality ratio for bubonic plague is about 30–60%.

If a person does not receive treatment for bubonic plague, it can spread to their blood. From there, the bacteria can spread quickly around a person’s body. This can cause a severe condition called septicemic plague. This form of plague is often fatal.

One place untreated bubonic plague can spread to is a person’s lungs. This can cause a person to develop pneumonic plague, which is more severe than bubonic plague. Without treatment, it is always fatal.

The following methods may help a person reduce transmitting plague-causing bacteria:

  • limiting potential rodent habitats in the yard by removing rock and wood piles, brush, and trash
  • keeping pet and wild animal foods in sealed containers
  • wearing gloves when handling live or dead animals that may have plague
  • contacting the local health department about disposal of dead animals
  • using insect repellent containing diethyltoluamide (DEET) when doing outdoor activities, such as working outside
  • using flea control products on pets and getting veterinary care if pets become sick
  • preventing dogs or cats that roam freely in areas where plague is present to sleep on the bed

Bubonic plague is an infectious disease that the bacteria Yersinia pestis causes. It has been around for thousands of years and is still present in many parts of the world.

The most common way for a person to contract Yersinia pestis and develop bubonic plague is via a fleabite. Additionally, a person may contract the bacteria if they handle fluids or tissue from an animal with the disease.

Bubonic plague can lead to a person developing buboes. These can become pus-filled open sores as the disease advances.

A healthcare professional can typically prescribe antibiotics to treat bubonic plague. Without treatment, the disease can develop into more severe forms of plague.

A person can help prevent Yersinia pestis transmission in various ways, including applying flea control products on their pets and keeping their yard free from potential rodent habitats.