﻿STUDIES IN PLAGUE IMMUNITY. 317 



Second, the Indo-China strain which, in contrast to the first, is distin- 

 guished by — 



(1) An almost invariable connection with great mortality among rats and 

 less frequently among other animals; (2) less frequent occurrence of epidemics 

 of Pestis minor, although it is possible that further observations may show that 

 these are commoner than has been supposed; (3) remarkable power of extension 

 as shown in its spread over a large part of India and conveyance to many other 

 countries; (4) in general, more intense virulence and higher case mortality; 

 (5) the pneumonic form accompanied by haemoptysis is much commoner. 



However, the variations in virulence of the disease are dependent 

 somewhat upon the differences in the susceptibility of the individual 

 attacked and are not entirely connected with the degree of virulence of 

 the organism, which as we have seen from a discussion of this subject, 

 may change under some of the physical conditions it meets in nature 

 and in certain of the passages through the animal body. 



Therefore, it must unquestionably be admitted that the pest bacillus 

 may under certain conditions become attenuated many times during the 

 course of an epidemic, and under certain others regain its virulence. 

 Nevertheless, the general stability of its virulence must be recognized, and 

 I believe that it is this quality which may markedly influence the nature 

 and course of at least some epidemics of plague. 



The plague organism in nature is not very resistant and easily becomes 

 destroyed under certain conditions, but it frequently does not easily 

 become attenuated, either in the animal body or outside of it. 



RELATION BETWEEN THE VIRULENCE OE THE PLAGUE BACILLUS AND ITS 



IMMUNIZING POWER. 



The question of the relation between the virulence and the immunizing 

 power of different strains of the plague bacillus obviously is an im- 

 portant one in its bearing upon the subject of protective inoculation, 

 particularly with reference to that form in which the living, attenuated 

 organism is employed. The subject of virulence in relation to the 

 immunizing power of microorganisms in general has attracted consider- 

 able attention during the past few years. Pfeiffer, Friedberger 11T and 

 myself 118 found that with cholera spirilla a greater immunity was 

 obtained with the more virulent organism. Pfeiffer and Friedberger 

 employed four strains in their investigations. My experiments were 

 carried on with two strains of cholera spirilla of widely different viru- 

 lence, and I was able conclusively to show that the virulent organism 

 upon inoculation produced a higher immunity and at the same time 

 bound a greater number of amboceptors in a cholera immune serum 



117 Berl. Klin. Wchnsch. (1902), 39, 581. 



118 Publications of the Bureau of Government Laboratories, Manila (1904), No. 

 21 ; 1 J. Exp. Med. (1905), 7, 229. 



