Plague is a zoonotic disease caused by the bacterium Yersinia pestis which is transmitted from rodent to rodent by infected fleas.  Plague outbreaks occur among rodents such as prairie dogs, ground squirrels, rats, and mice.  Plague exists naturally through a complex flea-rodent cycle.

Plague is characterized by periodic disease outbreaks in rodent populations, some of which have a high death rate.  During these outbreaks, hungry infected fleas that have lost their normal hosts seek other sources of blood, thus increasing the increased risk to humans and other animals frequenting the area. 

Epidemics of plague in humans usually involve house rats and their fleas.  Plague can be a very severe disease in humans, with a fatality rate of 50 percent if left untreated.  Humans can contract the disease through a flea bite, direct contact with plague-infected rodents, or direct contact with affected non-rodent hosts such as rabbits and cats.  Because of the risk of plague and other zoonotic diseases, private ownership of prairie dogs is illegal in the United States.  Modern antibiotics are effective against plague, but if an infected person is not treated promptly, the disease is likely to cause illness or death.

The typical sign of the most common form of human plague is a swollen and very tender lymph gland, accompanied by pain. The swollen gland is called a "bubo" (hence the term "bubonic plague"). Bubonic plague should be suspected when a person develops a swollen gland, fever, chills, headache, and extreme exhaustion, and has a history of possible exposure to infected rodents, rabbits, or fleas.