442 PLAGUE. 



There can be little doubt, however, that rats play the most 

 important part in distributing it over wide areas of a 

 town or district when once it has broken out. This has 

 been abundantly proved in the case of Bombay, where 

 observations have shown that the migration of plague- 

 infected rats to districts comparatively free from the disease 

 has been attended by extensive outbreak in these places. 

 The disease can also be transmitted by contagion from 

 persons affected, but this method of transmission can be 

 much more easily controlled than the previous. The bacillus 

 enters the human body by lesions of the skin surface, by 

 the respiratory passages, and possibly also by the alimentary 

 canal. The first mentioned is the commonest mode, cracks 

 and abrasions of the skin apparently supplying the means of 

 easy entrance ; in this connection the fact that there may 

 be no local lesion at the site of inoculation is of much im- 

 portance. The occurrence of infection by the respiratory 

 passages is less common, but is proved by the anatomical 

 changes as detailed above. In addition to the sputum, bacilli 

 have been demonstrated also in the urine and the faeces of 

 those suffering from the disease. From the facts stated with 

 regard to the powers of comparatively rapid multiplication 

 of the bacillus, its wide dissemination by affected rats, human 

 excreta, etc., it may be understood how extensively the 

 soil and dwellings may become infected, and how difficult 

 it may be to arrest the ravages of the disease. How 

 important a part such infection of a locality plays is strikingly 

 shown by the rapid fall in the number of cases where the 

 people go into tents. 



Immunity. Yersin, Calmette, and Borrel succeeded in 

 producing a certain amount of immunity in rabbits against 

 the organism by injection of cultures killed by heat at 

 58 C. They further found that the serum of such animals 

 had certain protective powers when tested in mice. 

 Later, they immunised a horse by intravenous injection 

 of the living bacilli, and obtained a serum which had 

 more powerful properties. This anti-plague serum has 

 been employed by Yersin in cases of the disease at Canton, 



