SCIENTIFIC BASIS OF SERUM THEORY 321 



immunity to these conditions. Accordingly, this type of resist- 

 ance against certain diseases is known as natural immunity. 



In some few cases an animal is born with a resistance to a 

 certain disease which may have attacked the sire or dam. In such 

 cases the young animal has conferred upon it a certain amount 

 of germ-resisting power against the particular germ that produced 

 disease in the parent. An immunity of this type, derived directly 

 from one or the other parent, is an hereditary acquirement, and is 

 classed as hereditary immunity. This form of protection is un- 

 common and is often only temporary in nature, as in the case of 

 young pigs born from mothers that have been through an attack 

 of cholera. 



In other even more rare cases the unborn animal suffers from 

 an attack of some disease while still in the uterus of the mother. 

 In such cases the pig is born with a protective power against this 

 particular disease. Such an immunity as this, obtained as a result 

 of an attack of the disease while in the womb of the mother, is 

 known as congenital immunity. 



Unfortunately for hogs, they are not naturally immune to 

 hog-cholera, and hereditary immunity to this disease is also com- 

 paratively rare. In those cases where an hereditary resistance is 

 inherited, it is comparatively weak, and after the pig is weaned 

 the protection lapses and is no longer of any value. Congenital 

 immunity to cholera is so rare as to be hardly worthy of mention. 

 In nearly every case where the pigs in the womb of the mother are 

 attacked by cholera the sow aborts and slinks the entire litter. 



Acquired Immunity. — Through a valuable provision of nature 

 it becomes possible for an animal born susceptible to a certain dis- 

 ease to afterward obtain a protective power or immunity against 

 this same disease and the germs which produce it. Coming as it 

 does after birth of the animal, and being something that is acquired 

 as the result of experience, it is spoken of as an acquired immunity. 

 We understand, then, by an acquired immunity a protective power 

 against a certain disease which is acquired or received after the 

 birth of the animal. It differs in this respect from natural, heredi- 

 tary, or congenital immunity in that all of these types are obtained 

 as a result of some influence affecting the animal body before birth. 

 Acquired immunity is the result of influences met with after birth. 



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