THE CAUSE OF INFECTIOUS DISEASE. 22Q 



of disease, in the internal constitution of 

 the fermentable substances, while Pasteur con- 

 ceived the cause to be the external and visible 

 yeast-cells. This conflict of opinion is now 

 still further complicated by Pettenkofer's theory 

 that, at least for certain diseases such as 

 typhoid fever and cholera, the determining 

 cause or essence is to be found in the external 

 conditions which vary according to time and 

 place. Here we have put forth as the true 

 and sufficient cause of epidemic disease three 

 wholly dissimilar things, upheld by three dif- 

 ferent schools, and which are all treated as 

 " entities " or personifications. Virchow finds 

 an internal cause in the diseased cells, his op- 

 ponents see an external cause in the germs 

 that bring about disease, and Pettenkofer sees 

 a cause in those external conditions which 

 play no particular role either in the eyes of. 

 Virchow or in those of Virchow's chief oppo- 

 nents. At the same time, however, it must be 

 said that Virchow consistently attempts to 

 break free from the personification idea and 

 to arrive at some conception based on the 

 comprehension of processes. 



I shall now attempt to show what is false in 

 each of these conceptions and what is scien- 

 tifically tenable, and in so doing I shall point 

 out that all these investigators recognized a 



