14 THE PRINCIPLES OF IMMUNOLOGY 



The microparasites, therefore, can be said to have a ready adapta- 

 bility to varying environment and to be aided in propagation by 

 their ability to derive nutrition from food-stuffs which possess 

 wide differences in constitution. Certain bacteria apparently can 

 produce their own protein from amino-acids and have no diffi- 

 culty in deriving nutrition from whole proteins. As has previ- 

 ously been indicated, those factors which go to increase virulence 

 of organisms, such as capsule formation and the production of toxic 

 substances, aid materially in invasion. 



Factors Inhibiting the Invader. Although rapid multiplication 

 aids invasion, nevertheless, the brief life period which most of the 

 microparasites exhibit is an influence operating against rather than 

 in favor of invasion. Many pathogenic organisms are susceptible 

 to the destructive influence of light, heat, desiccation, etc. In cer- 

 tain instances the life of organisms outside an animal body operates 

 to reduce virulence and therefore to inhibit the capacity for inva- 

 sion and production of disease. 



Factors Operating Against Resistance. The animal host is sub- 

 jected to the attacks of invading organisms because of the multi- 

 plicity of contacts with the environment. The large body surface 

 and locomotion of the body are influences favoring approximation 

 of the invader. Certain living activities, such as the ingestion of 

 foods and water, coitus, and the ready availability of superficial 

 orifices, such as the nose, ears, mouth, anus, genital orifices, all aid 

 invasion. The fact that most animals have a constant body tem- 

 perature and that their tissues are continually moist, provides con- 

 ditions favorable to the invading organism. Although light rays 

 beyond the violet end of the spectrum have a certain capacity for 

 the penetration of tissues, yet ordinary sunlight exhibits very little 

 penetrability; therefore, the construction of the body is such that 

 the inhibitory effect of light is not brought to bear upon organisms 

 that have already gained entrance. The anatomy of the body pro- 

 vides certain structures which are relatively inactive, such as the 

 appendix vermiformis and the crypts of the tonsils where organ- 

 isms find moisture, warmth, and darkness, suitable for their de- 

 velopment. In chronic infections, particularly by the tubercle 

 bacillus, necrotic tissues, or actual cavities may exist in contact with 

 surfaces and with the outer air, and both conditions operate to re- 

 duce resistance by providing favorable places for bacterial multipli- 

 cation. The circulation of lymph and blood may operate against 

 the host if organisms are particularly virulent. Inspiration of con- 

 taminated air may also serve to aid invaders. The resistance of the 

 host may, in a manner as yet unexplained, be decreased in general 

 by bodily fatigue, exposure to heat and cold, poor hygienic sur- 

 roundings, deleterious gases, or improper diet. The extremes of 

 life, childhood and age, are associated with reduced resistance. 

 Drugs, operative procedures, improper diet, and similar conditions 

 favor infection. 



Factors Favoring the Host. The possession of intelligence by 



