The Black Death “No plague has ever been so deadly, nor so hideous. Blood was its avatar and its seal: the redness and horror of blood. (Edgar Allen Poe's The Masque of the Red Death.) Many believed that the Black Death was a curse from God; punishment for the sins the infected had committed. Those who survived were the chosen people, those who respected the laws of the Church. Scientists now know that this devastating illness was not the result of sins or spiritual inadequacy, but that this terrible disease was caused by a strain of bacteria called Yersinia pestis. The bacteria were carried by rat fleas, which were very abundant in medieval towns due to unsanitary conditions and overcrowding. Fleas bit rats and became infected with diseased blood. The fleas then jumped from the rats to humans, thereby infecting the host. Because the plague spread easily, through sneezing and coughing as well as fleas, the infection spread like wildfire. By the end of the 1300s, more than a third of the populations of Europe, Asia, and Africa were completely wiped out. The Black Death was by far the deadliest disease known to man. It spread and killed with such virulence that the course of human history was changed forever. Little known to ordinary people, three forms of the Black Death existed. All were caused by the same bacteria, but each had very different symptoms. All three forms, although less deadly, killed millions of people in the Middle Ages. The most common and best known strain of the Black Death was the bubonic plague. Victims were prone to enlarged and inflamed lymph nodes, a characteristic known as a bubo, hence the name bubonic plague. The lymph nodes would swell to enormous capacities until they burst. Other symptoms included headache, nausea, joint pain, high fever and vomiting. Symptoms usually took about a week to appear, and the mortality rate was about 30 to 75 percent. The second form of the Black Death was the pneumonic plague. It was the second most commonly seen form of disease, although it was not as widespread as the bubonic plague. Many victims died before being able to infect others. This form of plague attacked the lungs. Viscous mucus tinged with blood came out of the mouth, and as the disease progressed, the sputum became thin and bright red..