The Plague

The plague has probably caused more fear and terror than any other disease in history. It has killed more than two hundred million people. But it also brought about some large changes, such as the end of the Dark Ages and the advancement of clinical research.
The plague is caused by a bacteria, Pasteurella pestis. The bacteria is originally carried by infected rats, which are bitten by fleas. The bacteria completely fills the flea's stomach so it can't digest any blood. Since the flea is so hungry, it bites a human anyway. …