BACILLUS PESTIS 241 



and infect the surrounding tissues. The pneumonic types of 

 plague may dev.elop after infection by a flea if the organisms enter- 

 ing the blood stream happen to become localized in the lungs. 



Immunity. One attack of plague as a rule confers immunity. 

 A method of protective inoculation with " Haffkine's prophylac- 

 tic " is extensively practiced in India which has given very good 

 results. The " prophylactic " is prepared by inoculating broth 

 cultures with B. pestis, and as soon as the stalactite formation 

 appears the tube is thoroughly shaken until the growth falls 

 to the bottom. The tube is then reinoculated with fresh growth 

 and the shaking and reinoculation process is repeated five or six 

 times. After about six weeks the culture is killed by heating 

 for one hour at 65 C. The dose administered is about 3 to 3.5 

 c.c., an amount equivalent to about 500 million bacteria. Vac- 

 cination gives a relatively short immunity but is sufficiently marked 

 to warrant its use in case of exposure to infection. In the Punjab 

 during the plague season 1902-1903, of those inoculated the mor- 

 tality was only 23.9 per cent as compared with 60.1 per cent 

 among the uninoculated. 



A serum prepared by Yersin by injecting horses first with killed 

 cultures and later with living cultures of B. pestis is reported by 

 certain investigators to have a curative action ; others, however, 

 have failed to secure any favorable result from its use. 



