Black Death: Unveiling the Deadly History and Pathogenesis of the Plague

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Throughout history, there have been many pathogens that are well-known. One of the best-known pathogens is Yersinia Pestis (Y. pestis), more commonly known as the Plague. There are three basic forms of Plague: bubonic, septicemic, and pneumonic. Each form has similar symptoms with small variations, and the mortality rates vary.

Y. Pestis Pathogenesis

The Y. pestis pathogen is a coccobacillus that, when stained with Giemsa, Gram, Wayson’s, or Wright, is a gram-negative pathogen. Y. pestis looks like a safety pin, which makes it easy to identify. It comes from the Enterobacteriaceae family. Y. pestis must be contained in blood if it is to survive. Y. pestis is usually transferred from an oriental rat flea, but it can be transmitted by rats. When the Y. pestis bacteria enters the flea, it backs up the flea’s digestive system. This causes the flea to feel hungry, and it feeds on humans. When it does this, the bacteria is then transferred to the human. While in the flea, the outer layer of the pathogen is lost. Once in the human host, the immune system kills almost all the organisms by polymorphonuclear leukocytes. The tissue macrophages take a few of the bacilli, but they can not kill the Y. pestis. They, therefore, can not create the correct antibodies to fight off the bacteria.

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The Y. pestis then spreads to the lymph system and quickly infects them. Many of the symptoms mimic the common flu, which makes it hard to diagnose. In Bubonic Plague, symptoms include high fever, general malaise, buboes (enlarged lymph nodes, usually in groin or neck areas), pain and tenderness in lymph node areas, septicemia, shock, convulsions, necrosis due to hemorrhagic changes, and cyanosis (why they call plague “black death”). Pneumonic Plague has the same symptoms with the inclusion of the lungs turning to liquid and causing the infected individual to cough up the lungs. Septicemic is the deadliest of these three due to it not only having the symptoms of both bubonic and pneumonic Plague, but the body loses the ability to clot. This means that the victim will bleed from all their orifices. (Chamberlain, N. R., & KCOM).

Yersinia Pestis throughout history

There have been three pandemics that have been the most devastating throughout history. The first pandemic lasted from approximately the 6th to 8th AD; this was named the Justinian Plague. The primary outbreak was located in the Mediterranean and eastern areas of the Roman Empire. It killed an estimated 25-50 million people, but that number could be higher. Similar to the outbreaks that followed, this pandemic is attributed to trade routes and changes in climate. The combination of these two factors is attributed to a heightened rat and flea population and an increase in poverty (Horgan, 2015). The second is more commonly known as the Black Plague, which ended the Dark Age. This one lasted from approximately the 14th – 15th century AD.

There are many controversies as to the exact number of people that it killed. They range from 25% to 60% of the European population. This is because the accounts differ between experts, and many are on the more conservative side of things. The last pandemic started in China in the 1860s and lasted until about the early 1900s, and it killed about 10 million people. This outbreak was transported then to the United States, which allowed for other species of animals to carry the disease. These include but are not limited to, squirrels, prairie dogs, chipmunks, and voles. Within the last five years, there have been more recent outbreaks in Madagascar, Africa, and South America. (Centers for Disease Control and Prevention -CDC 2015)


Yersinia Pestis is one of the deadliest pathogens there is. During recorded history, Y. pestis has been responsible for some of the most devastating losses of the human population. Y. pestis is a pathogen that can never really be eradicated if there is poverty and unclean practices. The number of deaths has decreased thanks to the discovery of antibiotics. This is solely dependent on whether the correct form is correctly identified so the antibiotic can be administered in a timely manner.


  1. Chamberlain, N. R., & KCOM. (n.d.). Yersinia pestis. Retrieved from
  2. Centers for Disease Control and Prevention – CDC. (2015). Plague. Retrieved from
  3. Horgan, J. (2015). Pandemic: The Black Death. History Extra. Retrieved from

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Black Death: Unveiling the Deadly History and Pathogenesis of the Plague. (2023, Aug 29). Retrieved from

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