276 SWINE 



never yet been successfully cultivated by any artificial 

 method. So far as known this virus will not produce the 

 disease in any other animal except the hog, at least not to 

 a fatal degree. 



Since this disease is prevalent to a greater or less ex- 

 tent only in the corn belt, it might be called a corn belt 

 disease and its prevalence depends apparently on the con- 

 dition of the animal. Nature has endowed all animals 

 with a means of protection against disease germs. The 

 white blood corpuscles of the body perform this duty by 

 destroying disease germs as they come into the system. 

 If by any system of feeding the general tone or vitality 

 of the animal has been reduced to such an extent that it is 

 unable to destroy the disease germs as they are brought 

 into the system, or if these germs are brought in in such 

 large numbers that the natural means of the animal with 

 which it is endowed cannot destroy them, then the dis- 

 ease will develop. Since the prevalence of the disease 

 is only in the cornbelt it would seem that this reduced 

 state of vitality of the animal body is entirely responsible 

 for its origin. After the disease once becomes estab- 

 lished, then the germs will be produced in such large 

 numbers that even the most resistant of animals, except 

 those that are naturally immune or have been rendered 

 immune artificially, will contract the disease. Herds that 

 have been properly fed, however, will not suffer so se- 

 verely as those that have been improperly fed. 



MANNER OF SPREADING THE DISEASE. 



The disease may be spread by association, which prob- 

 ably means the inhalation by a healthy individual of the 

 expelled air of a diseased animal ; also by ingestion of dis- 

 eased particles of food, or of virus direct. Any means 



