The plague is a general term that refers to any contagious epidemic disease, but usually refers specifically to bubonic plague (which gets its name from the swelling of the lymph nodes, or buboes). A bubonic plague epidemic spread throughout Europe and Asia in the middle of the fourteenth century, killing as much as 75 percent of the population in 20 years; that epidemic came to be known as the Black Death.

An acute infectious disease, the bubonic plague is carried to humans by fleas that have bitten infected rats and other rodents. Human symptoms include high fever, chills, swelling of the lymph nodes, and hemorrhages. Once the bacteria spreads to the lungs, it is quickly fatal. (This form of the disease is called pneumonic plague and can be transmitted from person to person via droplets.)

Improved sanitation, chiefly in developed nations, has reduced the occurrence of the disease. Bubonic plague still occurs, but the development of antibiotics in the twentieth century has greatly reduced the mortality rate.