The Bubonic Plague
It is a bacterial infection caused by Yersinia pestis.
Without treatment, the bubonic plague kills about two thirds of infected humans within four days.
The first recorded epidemic ravaged the Eastern Roman Empire and was named the Plague of Justinian after emperor Justinian I, who was infected but survived through extensive treatment.
In the Late Middle Ages (1340–1400) Europe experienced the most deadly disease outbreak in history when the Black Death, the infamous pandemic of bubonic plague, hit in 1347, killing a third of the human population.
The plague resurfaced for a third time in the mid-19th century. Like the two previous outbreaks, this one also originated in Eastern Asia. The initial outbreak occurred in China's Yunnan province in 1855. The disease remained localized in Southwest for several years before spreading.
Bubonic plague was important because it was a very harmful and dangerous disease and it killed a lot of people as a result.
In the story "The Room on the Fourth Floor" Mrs. Farringham died from this deadly disease and it made a very unexpected ending to it.