The Bubonic Plague, or Black Death, was one of the worst cases of viral pandemic known to man and occurred in the 14th century through the 17th century.
For 300 years, the Black Death swept across Europe repeatedly, killing millions of inhabitants, approximately 1/3 of Europe's total population at the time, as well as a 50% mortality rate in some crowded urban areas. The virus is none other than the bacillus Yersinia Pestis, which inhabits the blood of fleas, and is received by the fleas from black rats, pests common to Europe at the time. Once the flea carries the virus, its digestive system could no longer process the blood of the black rat, causing the fleas, starving and voracious for blood, to find another food source. Thankfully, the virus is not contagious to humans, but the Bubonic Plague was so widespread due to the amount of black rats and fleas present and numerous in that age. The flea could hide for up to 50 days hiding in grain or cloth, something that was extremely well-traded in Europe during those 300 hellious years. but the virus was not alone; it had a partner, the pneumonic plague. This was a disease contacted by humans and spread by humans. It attacks the lungs; they fill up with a thick liquid, making it harder to breathe. Pneumonia is still found today, but not as deadly.
The mortality rates for bubonic plague are 50 - 90% untreated, meaning no medicine, no healthy habits to fight it out, nada. Treated and caught early on, the rate drops to a mere 15%.