What was known as the Black Plague began in the East, in countries such as India, Tartar, Syria, and Armenia. It included at least four variants and was caused by the bacillus Yersina pestis. This bacillus thrives in the stomach of fleas which in turn are typically parasites of black rats. Due to circumstances not yet understood, the bacilli may multiply to such an extent that the flea's digestive tract is blocked and the flea regurgitates numerous bacilli into the bloodstream of the rodent host. The death of the rodent will cause the relocation of the flea, and if its next host is a human, then a contagion will begin.
The most common form of the disease was the bubonic plague, which caused hemorrhages or buboes of varying sizes. This is the least deadly, and is strictly insect borne; it cannot be transmitted from human to human. The pneumonic plague occurs when the bacillus moves into the pulmonary system, and generally occurred only when the disease was contracted in winter. The septicemic and enteric plague attack the blood and digestive systems, respectively, and are 100% fatal. These both are more rare than the other two types, perhaps because the hosts are killed so rapidly that the bacilli are not given much of a chance to thrive and be transmitted.
(Ed: D.S.) Courie, Leonard W. The Black Death and Peasant's Revolt. New York: Wayland Publishers, 1972; Strayer, Joseph R., ed. Dictionary of the Middle Ages. New York: Charles Scribner's Sons. Vol. 2. pp. 257-267.
Update 2012, a note by Monica Green, Professor of History, School of Historical and Religious Studies, Arizona State University, received on Sept. 17, 2012. Prof. Green is the organizer of a major session at the last American Historical Association annual meeting (New Orleans, January 3–6, 2013) entitled: "Re-Mapping the Black Death."
She writes: "Plague studies are in the midst of being revolutionized because of work in genomics. On the one hand, phylogenetic work is able to reconstruct the history of the pathogen, Yersinia pestis, itself. This does indeed show that the organism evolved in central Eurasia, but it's by no means clear precisely where. The main work on this matter is: Giovanna Morelli, et al., “Yersinia pestis Genome Sequencing Identifies Patterns of Global Phylogenetic Diversity,” Nature Genetics 42, no. 12 (December 2010), 1140-45. Unfortunately, there are some claims in this essay that no historian would agree with: e.g., that there were plague epidemics in "China." The assumption is that China has always had the boundaries it does currently; however, the place in central Eurasia where the origin of the plague bacillus would be located was never within the confines of what constituted "China" in the Middle Ages. For a very helpful guiding hand through all that has happened over the past decade and a half in this area of research, see: Lester K. Little, “Plague Historians in Lab Coats,” Past and Present 213 (2011), 267-90. On the mapping question in general, there is a superb article from last year that deconstructs the whole "mapping" question: David Mengel, “A Plague on Bohemia? Mapping the Black Death,” Past and Present no. 211 (May 2011), 3-34."