MICROBIAL DISEASES OF MAN AND ANIMALS. 655 



The etiology of hog cholera has long been the subject of scientific con- 

 troversy, but it is now generally acknowledged that the cause of this 

 disease is an invisible microorganism which exists in the blood, the internal 

 organs, and the urine of infected hogs. The fact that this disease is 

 caused by an invisible microorganism was demonstrated as follows:* 



FIG. 115. Hemorrhagic points on kidneys of hog-cholera. [J 



hog. (Original.) 



The blood serum of hogs infected with hog cholera acquired in the 

 natural way is very infectious for non-immune hogs, the disease being 

 readily transmitted by the subcutaneous injection of small amounts. 

 The disease produced by this subcutaneous injection is identical in all 

 respects with the disease as it occurs in nature. If, now, this infectious 

 serum is diluted with normal salt solution or with ordinary bouillon (i to 10) 

 and passed through either a Berkefeld or Chamberland filter, the 



* Bulletin 72, Bureau of Animal Industry, U. S. Dept. Agriculture, 1905. 



