22 QUATERCENTENARY STUDIES IN PATHOLOGY 



later passages. In the plague bacillus we are dealing with an organism 

 of the group of hsemorrhagic septicaemiai. Such bacteria, when intro- 

 duced into the bodies of certain animals, are known to lose their 

 virulence progressively. They give rise to outbreaks of spontaneous 

 disease in animals, and, on recrudescence of such epizootics, the 

 virulence of the organism becomes increased. During the natural 

 evolution of the epizootic, we find that the microbe becomes 

 attenuated, and either finally disappears, or, as in plague, persists 

 in chronic form. The recrudescence of rat plague in acute form would 

 appear to depend upon the natural infection of fresh generations of rats, 

 the bodies of which are highly susceptible to plague infection. My 

 observations and knowledge of the habits of rats lead me to the 

 conclusion that the obstinate clinging of plague to any particular area, 

 and its periodic recrudescence, is occasioned through chronic rat plague, 

 and further, the recrudescence of the epizootic occurs at certain seasons 

 of the year, namely, during the period of maximal rat prolificacy. 



(236) 



