Black death was an epidemic that spread to Western Europe and Britain in 1347–50. It caused havoc because about one-third of the European population died from the disease. The plague was weather related, because temperature and humidity were associated with multiplication of the carrier insects—Oriental rat fleas—which transmitted the bacteria from rats to human beings. After a flea fed on blood from the skin of an infected rodent, the ingested plague bacteria (Yersinia pestis) multiplied in the flea's upper digestive tract, blocking the flea's stomach. When the flea fed again on a human or another rodent, the blockage caused the freshly ingested blood to be regurgitated back into the bite, along with the plague bacteria. The infected human beings then carried these bacteria throughout their ...