Another question, “How did the first plague originate?” Plague is caused by the bacteria, Yersinia pestis. Modern genetic analysis has determined that Yersinia pestis is closely related to another bacteria, Yersinia pseudotuberculosis Yersinia pseudotuberculosis is capable of colonizing the hindgut of a flea but not other areas of the digestive system. It can be transmitted from flea to flea by a fecal elimination/ oral ingestion route. Transmission to humans requires that Yersinia colonize the midgut. Yersinia pestis produces a phospholipase D which Yersinia pseudotuberculosis does not. This change allows Yersinia pestis to colonize the midgut. Efficent transmission to the vertebrate host requires that Yersinia produce a biofilm in the mid and fore guts that blocks entry of blood. Yersinia pestis has lost 3 genes that are present in Yersinia pseudotuberculosis. The genes prevent biofilm formation. Their loss allows Yersinia pestis to produce biofilm and block the gut of the flea. The flea feeds more often because it is starving but regurgitates blood and Yersinia pestis because its gut is blocked. The changes in 4 genes are the difference between an infectious agent of minor importance and an epidemic disease that kills millions.
Yi-Cheng Sun, Clayton O. Jarrett, Christopher F. Bosio, B. Joseph Hinnebusch. Retracing the Evolutionary Path that Led to Flea-Borne Transmission of Yersinia pestis. Cell Host & Microbe. Volume 15, Issue 5, 14 May 2014, Pages 578–586.