Yersinia Pestis

Yersinia pestis is a zoonotic bacteria that is infamous for causing an epidemic that resulted in the death of more than 25 million people in Afro-Eurasia during the medieval period. Believed to have originated from China, Y.pestis was initially found in rodents infected with the bubonic plague. Despite being very weak in the face of heat and sunlight, this sneaky bacteria was able to rapidly spread from rodents to larger animals and humans with the help of pesky fleas that found shelter on these small creatures, making it a very, VERY unfortunate circumstance for the medieval people. Due to widespread poverty and the fact that proper hygiene and sanitation was nearly nonexistent during this time period, the plague was also contracted through the transfer of lice, the consumption of plague-infected animals, and direct contact with the body fluids of the infected.

On the surface of the body, plague patients could be seen with swollen lymph nodes and were often covered in gangrenous wounds. But what exactly is happening within the body? Once Y.pestis enters the wound, the bacteria quickly reproduces within the lymph nodes (which is why the nodes swell). Besides from the plague’s high mortality rate and its rapid transmission, another factor that makes this bacteria so horrific is its ability to invade and replicate within macrophages. By infecting these phagocytic cells, Y.pestis gains resistance to phagocytosis, which is the immune system’s process of eliminating foreign microorganisms. This in turn enhances the bacteria’s rate of development.

Although it is the most common form of the plague, the bubonic plague isn’t the most lethal form; the pneumonic plague, which occurs when Y.pestis enters the lungs due to the bubonic and/or septicemic plague being left untreated, is the plague’s most dangerous form. The septicemic plague is also quite lethal, as its location of infection is within the bloodstream.