CHAPTER XIV. 



THE ANTAGONISM EXISTING BETWEEN THE LIVING BODY 

 AND MICRO-ORGANISMS. 



THAT certain races of animals and men, and certain individuals 

 among these, are more refractory to disease than others is a fact which 

 has long been known. Experience and observation have taught us, 

 further, that the same individuals are at one time more resistant to 

 disease than at another. This inborn or spontaneous refractory con- 

 dition to an infectious disease is termed natural immunity, in contra- 

 distinction to that acquired by recovery from the disease. 



In bacteria, we distinguish between the ability to produce poison 

 and the power to multiply in the body. In animals and the higher 

 plants we distinguish between immunity to poison and power to 

 inhibit the development of bacteria. 



In regard to variations in susceptibility, certain known facts have 

 been accumulated. Thus, cold-blooded animals are generally insuscep- 

 tible to infection from those bacteria which produce disease in warm- 

 blooded animals, and vice versa. This is partly explained by the ina- 

 bility of the bacteria which grow at the temperature of warm-blooded 

 animals to thrive at the temperature commonly existing in cold-blooded 

 animals. But differences are observed not only between warm-blooded 

 and cold-blooded animals, but also between the several races of warm- 

 blooded animals. The anthrax bacillus is very infectious for the mouse 

 and guinea-pig, while the rat is not susceptible to it unless its body 

 resistance is reduced by disease and the amount of infection is great. 

 The inability of the micro-organism to grow in the body of an animal 

 does not usually indicate, however, an insusceptibility to its poison; 

 thus, for instance, rabbits are less susceptible than dogs to the effects 

 of the poison elaborated by the pneumococci, but these bacteria develop 

 much better in the former than in the latter. Differences in suscepti- 

 bility are sometimes very marked among different varieties of the same 

 race of animals, as, for instance, between different kinds of rats and 

 pigeons to anthrax. In animals, as a whole, it is noticed experimentally 

 that the young of all species are less resistant to infection than the older 

 and larger ones. 



The difficulty experienced by the large majority of bacteria in devel- 

 oping in the tissues of the healthy body can be to a great extent removed 

 by any cause which lowers the general or local vitality of the tissues. 

 Among the causes which bring about such lessened resistance of the 

 body are hunger and starvation, bad ventilation and heating, exhaustion 

 from overexertion, exposure to cold, the deleterious effects of poisons, 



