The causative agent of plague is a bacterium, Yersinia pestis .
Humans are incidental hosts and are usually infected by the bite of rodent fleas.
Plague may also be acquired by direct contact with infectious materials or by inhalation of infective respiratory droplets. Initial signs and symptoms of plague may be nonspecific, with fever, chills, malaise, myalgia, nausea, prostration, sore throat, and headache. Plague continues to be enzootic in wild rodent populations over large rural areas of the Americas, Africa, and Asia, with occasional outbreaks among commensal rodents in villages and small towns. Wild rodent plague poses a real, though limited, risk to humans. When infection spreads to rats in urban or populated areas, humans are at markedly increased risk of exposure.