An infectious disease of humans and rodents caused by the bacterium Yersinia pestis. The sylvatic (wild-animal) form persists today in more than 200 species of rodents throughout the world. At present, contact with wild rodents and their fleas leads to sporadic human disease. The explosive urban epidemics of the Middle Ages, known as the Black Death, resulted when the infection of dense populations of city rats living closely with humans introduced disease from the Near East. The disease then was spread both by rat fleas and by transmission between humans. During these outbreaks, as much as 50% of the European population died; occasionally, entire towns disappeared.