Bubonic plague is the best known manifestation of the bacterial disease plague, caused by the bacterium Yersinia pestis (formerly known as Pasteurella pestis). The term "bubonic plague" was often used synonymously for plague, but it does in fact refer specifically to an infection that enters through the skin and travels through the lymphatics, as is often seen in flea-borne infections. Bubonic plague kills about 50% of infected patients in 3–7 days without treatment, and is believed by many to be the Black Death that swept through Europe in the 1340s, killing millions.

The bubonic plague is an infection of the lymphatic system, usually resulting from the bite of an infected flea. The fleas are often found on rodents, such as rats and mice, and seek out other prey when their rodent hosts die. Once established, bacteria rapidly spread to the lymph nodes and multiply.

Yersinia pestis Bacilli can resist phagocytosis and even reproduce inside phagocytes and kill them. As the disease progresses, the lymph nodes can hemorrhage and become swollen and necrotic. Bubonic plague can progress to lethal septicemic plague in some cases.

The plague is also known to spread to the lungs and become the disease known as the pneumonic plague. The most famous symptom of bubonic plague is swollen lymph glands, called buboes. These are commonly found in the armpits, groin or neck.

The bubonic plague was the first step of the ongoing plague. Two other forms of the plague, pneumonic and septicemic, resulted after a patient with the bubonic plague developed pneumonia or blood poisoning.

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