The bacteriology of plague is almost the latest addition to the science. Kitasato, of Tokio, demonstrated the cause of plague to be a bacillus during the Hong Kong epidemic in 1894. This was immediately confirmed by Yersin, and further proved by the isolation in artificial media of a pure culture of a bacillus able to cause the specific disease of bubonic plague. The bacillus was first detected in the blood of patients suffering from the disease. It takes the form of a small, round-ended, oval cell, with marked polar staining, and hence having an appearance not unlike a diplococcus. In the middle there is a clear interspace, and the whole is surrounded with a thick capsule, stained only with difficulty. The organisms are often linked together in pairs or even chains, and exhibit involution forms. In culture the bacillus is even more coccal in form than in the body. The plague bacillus grows readily on the ordinary media at blood-heat, producing circular cream-coloured colonies with a wavy outline, which eventually coalesce to form a greyish film. The following negative characters help to distinguish it: No growth occurs on potato, milk is not coagulated, and gelatine is not liquefied; Gram's method does not stain the bacillus, and there are no spores; the bacillus is readily killed by heat and by desiccation over sulphuric acid at 30° C. Both in cultures and outside the body the bacillus loses virulence. To this may be attributed possibly the variety of forms of the plague bacillus which differ in virulence. On gaining entrance to the human body the bacillus affects in particular two organs, the spleen and the lymph glands. The latter become inflamed in groups, commencing frequently with those in the armpit (axillary) or groin (inguinal). The spleen suffers from inflammatory swelling, which may affect other organs also. In both places the bacilli occur in enormous numbers. Kitasato considers that the bacillus may enter the body by the three channels adopted by anthrax, namely, the skin, alimentary canal, and respiratory tract. Haffkine has prepared a vaccine to be used as a prophylactic. He grows a pure culture of Kitasato's bacillus in broth upon the surface of which some globules of fat ("ghee") have been placed. The bacillus grows upon this fat in copious stalactite form. From time to time this growth is shaken down, until after five or six weeks the shaken broth appears milky. The contained bacilli are killed by heating the fluid to 70° C. for one hour. The resultant is the vaccine, of which the dose is 3 cc. Haffkine believes that inoculated persons in India have suffered twenty times less than non-inoculated living under the same conditions.