198 PRINCIPLES OF VETERINARY SCIENCE 



of sufficient oxygen to destroy the particular invader. The 

 red blood cells act as carriers of the oxygen from the lungs. So 

 great is the oxidizing action of the blood that such virulent 

 pathogenic germs as the organisms which cause blackleg, when 

 inoculated directly into the blood stream, fail to produce disease, 

 yet they will cause death promptly if inoculated subcutaneously. 



The three agencies mentioned above are all concerned with 

 making conditions unfavorable for invaders. When the tem- 

 perature of the body is elevated it may be regarded as an indica- 

 tion that they are all active and the battle is progressing favorably. 

 The absence of fever, on the other hand, in cases of severe infec- 

 tion, may be an indication that the cells are overwhelmed by 

 bacteria and toxins. 



Long ago it was recognized by veterinarians that one attack 

 of certain diseases of animals generally prevented a recurrence. 

 It was also known that these diseases could be transferred only 

 to certain other animals. For centuries the reasons for these 

 natural phenomena were obscure and merely matters of specula- 

 tion. The beginning of our knowledge concerning the factors 

 responsible for these phenomena dates from 1796, when the 

 English physician Jenner had his attention drawn to it by a 

 woman patient, who told him that she could not take smallpox 

 because she had had cowpox. But it was not until the germ 

 theory of disease became an established fact and the development 

 of experimental research that the full significance and importance 

 of immunity as a means of controlling infectious diseases was 

 realized. 



Immunity is the general term used to express all of the factors 

 concerned with the resistance of the body to disease. In a 

 broad sense it may be thought of as that condition in which 

 an animal is protected against certain diseases that others readily 

 contract. The animal may be born possessing it, in which case 

 it is known as natural immunity; or the animal may develop it 

 after birth, when it is termed acquired immunity. So far as is 

 known, immunity is possible only against those diseases that 

 are caused by microorganisms or viruses. 



Natural immunity is inherited by the animal from its parents 

 and transmitted to its offspring. It may be an attribute of the 

 species, the breed, or the individual. Hog cholera, a typical 

 infectious disease, will serve to illustrate these three forms of 



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