ESSAYS ON BACTERIOLOGY. 55 



investigations for the attack upon the infectious dis- 

 eases, that they fall into two general groups: First, 

 those based upon the facts observed in the ordinary 

 course of the infectious diseases; second, those based 

 upon facts coming from other directions. What ex- 

 pectation or hopes open now before us in this great 

 field? 



If, as to the questions of self -limitation and im- 

 munity, we study the infectious diseases, we shall find 

 that they fall into three qtiite distinct classes. 



First, there is a class of these diseases in which both 

 self-limitation 'and immunity are practically absolute. 

 Such are measles, scarlatina, smallpox, typhoid fever. 

 This being true, we might hope to command the situ- 

 ation, so far as immunity is concerned, if we could 

 secure an attenuated virus or the chemical products 

 of their germs. It would hardly seem that we could 

 expect to accomplish much in the way of cure, even 

 by an attenuated virus, since by this process , we 

 should subject the already crippled body to something 

 of the harmful as well as helpful agents. But pro- 

 vided these agents are different, and we could obtain 

 the latter, we might then have settled even the matter 

 of cure. 



The second class includes diseases in which self- 

 limitation is absolute, but subsequent immunity is 

 slight, or wanting. Here we find such affections as 

 diphtheria, erysipelas and pneumonia. With these 

 the problem of cure would be the same as before. But 



