320 DISEASES OF SWINE 



SOENTIFIC BASIS OF SERUM THEORY 



In evolving the serum treatment for hog-cholera the early- 

 workers in the experimental field were prompted by the scientific 

 principles which are believed to underlie the protective power 

 which some animals have against disease. Every farmer knows 

 that cattle are not susceptible to hog-cholera, and will not become 

 sick, no matter how often they may be in the same feed lots with 

 hogs that have cholera. It has also been noticed in almost every 

 outbreak that there will be one or two animals in the herd that do 

 not seem to take the disease or, if they do, at least show very little 

 or no symptoms of being sick. 



More interesting still is the noticeable fact that after animals 

 have been through an attack of cholera, if they are not killed by the 

 disease, they are usually left with some strange resisting power 

 against cholera. As a result, they will not again take the disease, 

 even though they be exposed in the same pens with animals that 

 have the disease. This is a very interesting fact, and one that has 

 attracted attention for centuries, not only in the case of hog- 

 cholera, but in numerous other diseases of man and animal as well. 

 For instance, it has been noted for centuries that after a man has 

 had one attack of small-pox he is not likely to ever again have the 

 disease. 



This strange protective power that is left after an attack of 

 disease has been designated in scientific works as immuriity, and has 

 been the subject of much thought and study by some of the most 

 brilliant minds of the past as well as the present century. Many 

 plans have been devised to show why this should be so. None of 

 the theories advanced has ever proved entirely satisfactory. At 

 the present time the explanation advanced by Professor Ehrlich, 

 of Germany, is the one most favorably regarded. 



Varieties of Immunity. — This resisting power or immunity to 

 disease is of more than one kind. For instance, in the case of cat- 

 tle, all the animals of this species havfe a strong resistance against 

 hog-cholera. In like manner, they are not affected by swine plague, 

 chicken-cholera, and many other diseases that attack other domes- 

 tic animals. Calves are born with this immunity to the diseases 

 just mentioned, and it is just simply natural for them to have this 



