What Organisms Caused the Black Death?

The Black Death was a horrific pandemic that quickly spread throughout Europe, killing one-third of its population in the 14th century. Also known as the Black Plague, this historic catastrophe continues to intrigue people today. Learn about the deadly bacteria that caused the Black Death, how it was spread and how it devastated an entire continent to gain insight into one of history's most deadly pathogenic events.

  1. The Y. Pestis Bacteria

    • A deadly organism silently and rapidly infected its victims, who also passed the microbe virally to other people. A strain of bacteria known as Yersinia pestis is the organism responsible for all three forms of the plague: bubonic, pneumonic and septicaemic plague. Bubonic plague was the most common form of plague during the Black Death. Within a week of infection, black, swollen sores called buboes would appear on the victim's flesh, followed by hemorrhaging. The Y. pestis bacteria would multiply and overwhelm the victim's nervous system and ultimately result in death. A deadly combination of Y. pestis and cold weather resulted in pneumonic plague, wherein the victim suffered a pneumonia-like infection of the lungs. Though not as common as bubonic plague, pneumonic plague was much more contagious, since the Y. pestis bacteria was expelled into the air by the coughing victim, virally infecting others nearby. Septicaemic plague was the least frequent, but deadliest, form of plague. Septicaemic plague occurred when vast amounts of Y. pestis entered the victim's bloodstream. Death occurred rapidly, usually, within a day, before buboes could even have a chance to form.


    • The Y. pestis bacteria initially made its way into its victims through animal-to-animal transmission via rat fleas. Small fleas would bite rats infected with the Y. pestis bacteria. The infected fleas, brought into towns and villages by the hundreds across Europe on the backs of their rat hosts, would then bite and infect unaware humans. Trade exacerbated the problem and caused the plague to rapidly spread over a large area in a short amount of time because in the 1300s, trade ships and cargo infested with stowaway rats went from port to port, unknowingly bringing Y. pestis wherever ships were docked and unloaded. Rats bearing the infected fleas would travel on and off the ships and into town centers and private residences. Because of this process of transmission, fleas can also be considered an organism responsible for causing the Black Death.

    Other Methods of Infection

    • Though Y. pestis was most frequently passed to humans via rats, other animals also carried the disease and infected people in 14th century Europe. Mice, squirrels and other flea-carrying rodents brought fleas infected with Y. pestis into towns and villages, which, in 14th century Europe, were infested with these animals. A fatal mistake during the time of the plague was to remove a dead rodent from the household. The living fleas would immediately jump from their dead host to the warm human body, biting and infecting the victim.

    Contemporary Explanations

    • While it is now known that plague is transmitted to humans via bites from fleas infected with the Y. pestis bacteria, this fact was not discovered until the 19th century. At the time of the Black Death, no one suspected that Europe's rat infestation was responsible for killing so many people. Other explanations ranging from earthquakes to poisoned water were maintained. Some even believed that sins or evil elements in the body were to blame. While the Y. pestis bacteria continues to infect humans today, it is easily treatable with antibiotics.

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