The plague used to occur in gigantic epidemics in Europe, but till lately had been banished for many years. Its recent appearance in Hong-Kong and in India shows that it has lost none of its old virulence. The disease is characterized by the appearance of lymphatic glandular swelling, usually single at first and mostly in the groin. This bubo grows quickly and the patient generally dies in, two or three days. During the epidemic rats and mice are affected and die in large numbers.

A short thick bacillus with rounded ends, exhibiting bipolar-staining characters, leaving a clear middle part (like the bacillus of septicaemia haemorrhagica), has been found in the pus or debris of the bubo. It is readily stained by the ordinary aniline dyes but not by Gram's method. Sometimes the bacillus has a capsule. It is also present in the blood, but in much smaller quantities. Cultures have been made on agar-agar, bouillon, gelatine, etc. Inoculated in guinea-pigs, rats, and mice, it kills in a few days, and the bacilli are found in the lymphatic glands and in the blood.