Plague is a deadly infectious disease caused by Yersinia pestis. The pathogen has rodent reservoirs and is transmitted to humans usually by fleas as vectors. There are three major manifestations: bubonic, septicemic, and pneumonic plagues. Severe epidemics in the past include the Plague of Justinian in the sixth century, the Black Death in the fourteenth century, and the Third Pandemic that began in Central Asia in the nineteenth century. The genome sequences have been determined for the strains responsible for these epidemics.