Why and how did the bubonic plague begin?

== == * Plague is primarily a disease of rodents. Infection most often occurs when a person is bitten by a rat or flea that has fed on an infected rodent. The bacteria multiply inside the flea, sticking together to form a plug that blocks its stomach and causes it to begin to starve. The flea then voraciously bite a host and continues to feed, even though it is unable to satisfy its hunger. During the feeding process, blood cannot flow into the blocked stomach, and consequently the flea vomits blood tainted with the bacteria back into the bite wound. The Bubonic plague bacterium then infects a new host, and the flea eventually dies from starvation. Any serious outbreak of plague is usually started by other disease outbreaks in rodents, or some other crash in the rodent population. During these outbreaks, infected fleas that have lost their normal hosts seek other sources of blood. * The Pope (Gregory IX) effectively outlawed cats with a papal bulletin Vox in Rama (1232). Cats had been killing the rats, which hosted diseased fleas, which in turn over populated. Only the aristocracy kept their cats through this ban as many more vulnerable cats and citizens where burned at the stake as witches.