A person with the bubonic plague has a serious infection caused by the bacteria, Yersinia pestis.

The bacteria is transmitted to humans by rat flees, ticks and lice, but can also be spread from infected animals. The most common symptom is tender, swollen lymph glands. The bacteria can infect the skin, throat, lungs or brain. Bubonic plague is very rare: less than 10 cases of plague occur in the US each year.