Infectious fever caused by the bacterium Yersinia pestis , carried by the rat flea.
It usually spreads to humans only when the flea runs out of rodent hosts. It takes three forms. Bubonic, the mildest, has characteristic swollen lymph node s (buboes) and is spread only by the flea. It accounts for three-fourths of plague cases. Pneumonic plague has extensive lung involvement and is spread in droplets from the lungs; it is often fatal in three or four days without treatment. In septicemic plague, bacteria overwhelm the bloodstream and often cause death within 24 hours, before other symptoms have a chance to develop. In the 14th century, plague ravaged Europe and Asia and was called the Black Death . Plague does not respond to penicillin, but other antibiotic s are effective. Sanitary measures against fleas and rodents, quarantine , and extreme caution in handling infectious materials help to suppress epidemics. A vaccine can prevent plague.