Bubonic plague

Bubonic plague is a serious infectious disease caused by the bacterium *Yersinia pestis*.

It is one of three distinct types of plague. The other forms are septicemic plague and pneumonic plague. The specific type depends on how the bacteria enter the body. Bubonic plague is a major subject of study because it has shaped human history through massive pandemics. It remains a significant public health concern in specific parts of the world today.

The mechanism of infection often begins with a tiny parasite called a flea.

The Oriental rat flea, *Xenopsylla cheopis*, is a very effective carrier of the bacteria. Inside the flea, the *Y. pestis* bacteria form aggregates in the gut. This creates a biofilm that blocks the flea's foregut. When the flea tries to feed on a host, it regurgitates the infected blood into the bite wound. This process transmits the bacteria directly into the host's skin.

Once the bacteria enter the body, they travel through lymphatic vessels. They eventually reach a lymph node, which is a part of the immune system. The bacteria colonize and reproduce within the node, causing it to become inflamed and swollen. These swollen, painful lymph nodes are called buboes. They are most commonly found in the groin, armpits, or neck. In some cases, the infection can progress to septicemic plague, where bacteria enter the blood, or pneumonic plague, where they infect the lungs.

The symptoms of bubonic plague can be quite severe. Between one and seven days after exposure, a person may develop fever, headaches, and vomiting. The characteristic buboes can grow very large and painful, sometimes even breaking open. Another symptom is acral necrosis, which is the dark discoloration of skin on the fingers, toes, lips, or nose.

If the disease spreads to the lungs, patients may experience heavy breathing and coughing up blood. Without medical intervention, the mortality rate is between 30% and 90%.

History shows that *Y. pestis* has caused massive shifts in human civilization. The Black Death in the 14th century swept through Asia, Europe, and Africa. It killed an estimated 50 million people and took 25% to 60% of Europe's population. This loss of life caused wages to rise because there were fewer workers available.

Other major events include the Plague of Justinian in the 6th century CE. There was also a third epidemic that began in the Yunnan Province in 1855.

Modern medicine has changed how we manage this disease. Doctors use antibiotics like streptomycin, gentamicin, or doxycycline to treat infections. If treatment begins within 24 hours of the first symptoms, the risk of death drops to about 10%.

Public health officials also use pest control and sanitation to manage rat populations. In the past, cities used methods like quarantine or even machines to deliver sulphur dioxide to kill pests.

Plague is still documented in the modern world. Between 2010 and 2015, there were 3,248 documented cases globally, resulting in 584 deaths. The Democratic Republic of the Congo, Madagascar, and Peru report the highest numbers of cases.

In the United States, cases are rare but do occur in rural areas of states like Arizona and New Mexico. Understanding the relationship between humans, fleas, and rodents helps scientists continue to prevent new outbreaks.