HISTORICAL SKETCH 7. 



set forth by Metchnikoff and Ehrlich and vaccine therapy 

 which at the present moment is attracting much attention. 



Although the methods that have been introduced for 

 the study of specific etiology have resulted in finding the 

 cause of a very large number of infectious diseases, they 

 have not been sufficient to demonstrate the etiology of epi- 

 zootic hog cholera, rinderpest, foot and mouth disease, dog 

 distemper and other important plagues of the domesticated 

 animals. Many of these diseases have been found to be due 

 to agents that will pass through the Berkefeld or Chamber- 

 land filters and consequently the causes of these maladies, 

 are spoken of as filterable viruses. 



