The Collaborative International Dictionary of English v.0.48:

Bubonic plague \Bubonic plague\ (Med.)
   a severe and often fatal disease caused by infection with the
   bacterium Yersinia pestis (formerly Pasteurella pestis),
   transmitted to man by the bite of fleas, themselves usually
   infected by biting infected rodents. It is characterized by
   the formation of buboes, most notably on the groin and
   armpits, and accompanied by weakness and high fever. The
   disease was known as the black death, and was responsible
   for several devastating plagues throughout the middle ages.
   When lungs became infected, the disease was called the
   pneumonic plague. It is still found occasionally in poor
   areas of undeveloped countries but is rare in developed
   countries.
   [PJC]

WordNet (r) 3.0 (2006):

Yersinia pestis
    n 1: a bacillus bacterium that causes the plague; aerosolized
         bacteria can be used as a bioweapon