Plagues are a type of epidemic caused by the bacteria Yersinia pestis. This bacteria lives in rats and rodents and is usually transmitted by the fleas that live on them.  

Bubonic-The bacteria is in the lymph nodes and causes swellings called buboes. Most common form of plague.

Septicemic-The bacteria is in the bloodstream.

Pneumonic-The bacteria is in the lungs. It is extremely dangerous since coughing can spread it.  

The first recorded plague was told of in First Samuel in The Bible. It began about 1000 B.C. and was called the "Plague of Ashod." About 542 A.D. the first pandemic plague occurred. It struck in Constantinople (now Istanbul) and killed about half the population. The Black Death was the second recorded pandemic. It began in central Asia and was carried west to Europe by trading ships.    

Information on Black Death                  A plague pit, or mass grave.