104 GEOGRAPHIC DISTRIBUriOy OF BUBONIC PLAGUE 



matia in 1840 and in Constantinople in 1841. Egypt, wliich for 

 centuries had been tlie })rincipal focus from which })higue had been 

 introduced into Euro{)e, continued to suffer from the disease until 

 1845, when it disappeared from that country. 



The last a])pearance of oriental plague in Europe, until its recent 

 introduction into Portugal, was the outbreak on the banks of the 

 Volga in 187S-'79. The disease had previously prevailed in a mild 

 form in the vicinity of Astrakhan and was probably introduced from 

 that localit}'. An interesting fact in connection with this epidemic 

 is that in Astrakhan the disease was so mild that no deaths occurred, 

 and that the earlier cases on the right l)ank of the Volga were of the 

 same mild form, but that the disease there increased rapidl3^ in 

 severity and soon became so malignant that scarcely an}' of those 

 attacked recovered. Tliis is to some extent the history of epidemics 

 elsewhere, and not only of plague, but of other infectious diseases, 

 such as typhus fever, cholera, and yellow fever. In all of these dis- 

 eases the outset of an e])idemic maybe characterized by cases so mild 

 in character that they are not recognized, and during the progress of 

 the epidemic many such cases may continue to occur. These cases 

 are evidently especially dangerous as regards the propagation of the 

 disease, for when the}' are not recognized no restrictions are placed 

 upon the infected individuals, although they may be sowing the 

 germs broadcast. 



The termination of an epidemic in the presanitary period depended 

 to a considerable extent upon the fact that those who suffered a mild 

 attack acquired thereb}' an immunity, and that when the more sus- 

 ceptible individuals in a communit}' had succumbed to the prevail- 

 ing epidemic, there was a necessary termination of the epidemic for 

 want of material. This is illustrated in such cities as Havana and 

 Rio de Janeiro, where yellow fever is an endemic disease. The 

 natives of these cities have an immunity which probabl}' results from 

 their having suffered a mild attack during childhood, and the epidemic 

 l)revalence of the disease depends on the presence of " unacclimated " 

 strangers. 



Another factor which no doubt has an important ])earing 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 

 discuss this subject in its scientific and practical aspects, but the gen- 

 eral fact may be stated that all known disease germs may var}' greatly 

 in their pathogenic virulence, and that in every infectious disease 



