DISEASE IN GENERAL 199 



natural immunity. While hog cholera is usually fatal to swine, 

 horses possess a natural immunity common to the species against 

 this disease. An instance where a species possesses a general 

 immunity, but where individuals are occasionally attacked, is 

 seen in tuberculosis of the dog and cat. These animals are 

 usually immune to tuberculosis, but now and then one is found 

 affected with it. Breed immunity against hog cholera is claimed 

 by some unscrupulous breeders to be possessed by mule-foot 

 hogs. Experiments conducted to prove this claim have resulted 

 in the contraction of the disease by animals of this breed, so the 

 claim is completely disproved. That breed immunity does 

 exist is shown by the fact that Algerian sheep are naturally 

 immune to anthrax, yet all other breeds of sheep are highly 

 susceptible to it. An example of individual immunity is often 

 seen in outbreaks of hog cholera, in which a few hogs pass through 

 the infection without getting sick, although hundreds die. The 

 individuals in these cases must be above the average of the 

 species in immunity. It seems reasonable to attribute their 

 protection to such factors as "strong constitution" or "good 

 condition" rather than to specific bactericidal and other prop- 

 erties of their blood-serum. 



Acquired immunity is the result of some influence acting 

 after birth. It occurs after recovery from a natural attack of 

 disease or from injecting the animal with living, weakened, or 

 killed germs of disease or their products. When this is done, a 

 reaction similar to an actual attack of the disease takes place. 

 Prepared for use in this manner, the germs are known as vaccines 

 or bacterins and the process of administration is called vaccina- 

 tion. It may be thought of as a reinforcement of the body to 

 guard against disease. The result is active immunity which varies 

 greatly in length in different animals, but may be permanent and 

 last for life. The development of active immunity is slow, more 

 or less dangerous, and always attended with some discomfort. 

 Several doses of a vaccine or a bacterin properly graduated often 

 give better results than a single dose in attempts to 'confer 

 immunity. 



As has been stated, after an animal has recovered from many 

 infectious diseases, its blood contains substances that protect it 

 from future attacks. What is even more remarkable is that the 

 fluid part or serum of the blood from a recovered animal, when 



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