﻿STUDIES IN PLAGUE IMMUNITY. 315 



organism has come in contact with a number of particularly susceptible 

 individuals and the period of the epidemic, under certain conditions 

 at least, seems to be limited particularly by the virulence of the organism 

 and by the number of susceptible individuals which continue to be in- 

 fected with it. As the virulence of the organism diminishes, the number 

 of cases of cholera in a community will diminish. Avirulent cholera 

 organisms, when grown on culture media in the laboratory, are practicall}' 

 harmless when ingested by all but very susceptible human beings and 

 the same is probably true of the more attenuated spirilla which occur in 

 nature. It would then appear that the instability of the virulence of 

 the cholera organism must frequently play an important role in determin- 

 ing the character and duration of the cholera epidemic. 



How different is the picture in at least some plague epidemics, such, 

 for example, as that which has been witnessed in India for the past ten 

 years and in which the number of deaths reported each year has almost 

 steadily increased from 1897 to 1904-5. The reported deaths in India 

 from this disease, according to the Indian Medical Gazette of July, 1906, 

 are as follows : 



Deaths. 

 Up to end of 1897 57,965 



1898 118,103 



1899 134,102 



1900 91,627 



1901 273,679 



1902 577,427 



1903 851,263 



1904 1,022,300 



1905 '. 950,863 



1906 to end of April 170,000 



It can not be argued that the disease is becoming any more or any 

 less virulent from year to year, judging from the death rate, for we 

 know that it is still spreading and that it has not even yet extended over 

 the whole of India. Burma has escaped until within the last few months, 

 but is now suffering very severely. 



Knowing as we do from laboratory experiments that the plague bacillus 

 is frequently extremely stable in its virulence in nature and that it may 

 not easily become attenuated under artificial conditions, knowing also 

 that the epidemic of this disease in India has not decreased in a period 

 of ten years, the outlook for the extermination of the malady in that 

 country by any other means than by the exhaustion of suitable susceptible 

 individuals and animals does not now seem to be hopeful. 



It is for this reason that I agree with Professor Haffkine, Colonel 

 Bannermann and many others in the idea that it is certain that protective 

 inoculation against plague must become one of the most effective means 



55670 — 11 



