Bubonic plague causes

The major forms of plague are caused by Yersinia pestis living in rodents. The disease is then transferred to humans from the fleas that originally bit the infected rodent.

Causes
Plague is caused by the organism Yersinia pestis. Rodents, such as rats, spread the disease to humans. People can get the plague when they are bitten by a flea that carries the plague bacteria from an infected rodent. In rare cases, you may get the disease when handling an infected animal. Certain forms of the plague can be spread from human to human. When someone with pneumonic plague coughs, microscopic droplets carrying the infection move through the air. Anyone who breathes in these particles may catch the disease. An epidemic may be started this way. In the Middle Ages, massive plague epidemics killed millions of people. Today, plague is rare in the United States, but has been known to occur in parts of California, Utah, Arizona, Nevada, and New Mexico. There three most common forms of plague are:
 * Bubonic plague -- an infection of the lymph nodes
 * Pneumonic plague -- an infection of the lungs
 * Septicemic plague -- an infection of the blood
 * The time between being infected and developing symptoms is typically 2 to 10 days, but may be as short as a few hours for pneumonic plague.