320 SPECIFIC MICRO-ORGANISMS 



on man or at large in human habitations as long as their normal 

 hosts are at hand, but when the rats sicken and die of plague, 

 then the fleas leave and becoming hungry they bite human beings 

 and thus 'noculate them with plague bacilli. 



In its permanent endemic centers, plague exists as an acute 

 and chronic disease of rodents. It spreads from these regions 

 through the agency of the wandering rats traveling along the 

 routes of commerce and especially in ships. The infected rat, 

 arrived at its destination, sets up an epizootic among its own 

 species, which later spreads to other animals and to man through 

 the agency of fleas, producing the bubonic form of the disease. 

 The infection may then be transmitted from man to man by 

 fomites and directly by contact, and by infectious material sus- 

 pended in the air, giving rise to the pneumonic form of the dis- 

 ease. A persistent epizootic of chronic plague among rodents 

 in a new region may give rise to a new permanent endemic 

 center. 



In man the disease occurs in two principal forms, the bubonic 

 type, in which the portal of entry is on the skin or mucous mem- 

 brane and the disease is manifested by swelling of the neighboring 

 lymph nodes, and the pneumonic type in which the organisms 

 are inhaled or aspirated into the lung. Both of these forms re- 

 sult in general bacteremia, as a rule. The bubonic form is largely 

 due to inoculation of the skin by bites of insects (fleas), while the 

 pneumonic form is transmitted more directly. Other clinical types 

 of the disease occur. The death rate is 30 to 90 per cent in the 

 bubonic and 98 to 100 per cent in the pneumonic type. In the 

 bacteriological diagnosis, the morphology of the organism in the 

 tissues and in cultures, its effect upon rats and guinea-pigs, and, 

 finally, agglutination of the newly isolated culture with a known 

 immune pest serum are important points. 



Immunity, at least a relative immunity, follows recovery from 

 the plague. Artificial immunity can be induced by injection 

 of attenuated living cultures and by the injection of killed bac- 

 teria (Haffkine's method). Many modifications of the latter 



