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. The stomach can't hold the blood, so the flea regurgitates into the small open bite wound on the person. That person now is infected with plague.
There are three types of plague. The first type is the Bubonic plague. This is the form that caused the Black Death, which killed a considerable percent of the population of Europe.
The symptoms, which occur two to six days after infection, are painfully swollen lymph nodes (which are called buboes), black spots on the skin, a headache, and a fever. Death usually occurs about four days after infection.
The second type is the Septicemic plague. In this form, the lymph nodes are not initially involved, which makes the disease more deadly since it's hard to diagnose. Its symptoms are blood poisoning, shock, and hemorrhaging.
The last and most deadly type is the pneumonic plague. This form of plague is the most dangerous for a couple of reasons. First, it has an extremely high fatality rate (can be over 50%!). Second, it can spread through the air by droplets coughed up by infected people. Symptoms include serious pneumonia, fever, and chills. The incubation period is one to three days.
The plague has been responsible for three major pandemics and...