. (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 ]