Plague is caused by the bacterium *Yersinia pestis*. It is one of the most serious bacterial infectious diseases. *Yersinia pestis* is found in wild rodents and their fleas. Rats, squirrels, and prairie dogs are some of the most commonly infected rodents. Rabbits can also carry the bacteria. Plague is usually transmitted to humans through the bites of infected fleas. Direct contact with infected animal tissue can also cause plague. Pneumonic plague is a form of plague that is more easily transmitted from person to person. A healthy person can contract the disease by inhaling droplets expelled by a coughing pneumonic plague patient. The most important routes of plague transmission are:

* Flea bites
* Exposure to people with pneumonic plague
* Handling infected carcasses
* Scratches or bites from infected domestic cats
* Exposure to aerosols containing plague bacilli
* Another potential route of plague transmission in humans was reported in 2014, through contact with an infected dog.