B UBONIC PLA G UE 99 



lignant that scarcely any of those attacked recovered. 

 This is to some extent the history of epidemics else- 

 where, and not only of plague, but of other infectious 

 diseases, such as typhus fever, cholera, and yellow- 

 fever. In all of these diseases the outset of an epi- 

 demic may be characterised by cases so mild in char- 

 acter that they are not recognised, and during the 

 progress of the epidemic many such cases may con- 

 tinue to occur. These cases are evidently especially 

 dangerous as regards the propagation of the disease, 

 for when they are not recognised no restrictions are 

 placed upon the infected individuals, although they 

 may be sowing the germs broadcast. 



The termination of an epidemic in the pre-sanitary 

 period depended to a considerable extent upon the 

 fact that those who suffered a mild attack acquired 

 thereby an immunity, and that when the more sus- 

 ceptible individuals in a community had succumbed 

 to the prevailing disease there was a necessary termi- 

 nation of the epidemic for want of material. 



Another factor which, no doubt, has an important 

 bearing upon the termination of epidemics is a change 

 in the virulence of the germ as a result of various 

 natural agencies. Time will not permit me to dis- 

 cuss this subject in its scientific and practical aspects, 

 but the general fact may be stated that all known 

 disease germs may vary greatly in their pathogenic 



