CHAPTER II 



GENERAL CONDITIONS OF INFECTION AND 

 RESISTANCE 



THE PRODUCTION OF INFECTIONS. 



ENTRANCE OF THE INVADING ORGANISM. 



TYPES OF INFECTIOUS DISEASES. 

 FACTORS FAVORING THE INVADER. 

 FACTORS INHIBITING THE INVADER. 



FACTORS OPERATING AGAINST RESISTANCE OF HOST. 

 FACTORS FAVORING THE HOST. 

 THE COURSE OF ACUTE INFECTIOUS DISEASE. 



The Production of Infection. The widespread dissemination of 

 bacteria in nature is such that they have ready access to plants and 

 animals. Invasion by pathogenic forms may set up- infection. 

 Whether or not the infection may lead to disease depends upon the 

 final relationship established between the invader and the invaded 

 body. There is probably no condition under which animals or 

 plants fail to exhibit some degree of resistance to the invading 

 organism, and similarly the latter attempts to accommodate itself 

 to the conditions found in the invaded host. If the resistance be not 

 sufficient to overcome the invader, infection results. The produc- 

 tion of disease, however, depends upon the superior powers of the 

 invader over the resistance of the host. Occasionally a mutual 

 adaptation appears, under which circumstances an animal may be 

 infected by an organism, but shows no symptom or sign of dis- 

 ease. Not infrequently the trypanosoma Lewisi is found in the 

 blood stream of rats, the rats continuing to live an apparently nor- 

 mal existence. A similar mutual adaptation is found in the " carrier 

 state," wherein man may harbor virulent diphtheria bacilli or other 

 organisms without any evidence of disease. Mutual adaptation is 

 not attained without a struggle on the part of both invader and 

 host, and infectious disease results when the invading organism 

 triumphs. This does not mean permanence of infection, because 

 even although disease is established, the defenses of the host 

 continue to operate, and often are augmented in such a way that 

 ultimately the infection disappears. This accounts for the self- 

 limitation of most of the acute infectious diseases. The increase in 

 defensive powers may in certain diseases become permanent and 

 immunity thereby be established. In all cases of recovery from 

 acute infections immunity of some duration appears, although it may 

 be limited to a few weeks or a few months. 



Entrance of the Invader. The entrance of the invading organ- 

 ism may be due to an interruption of continuity of those surfaces of 

 the body which ordinarily are impermeable to bacterial invasion. 

 These surfaces include skin and the mucous membranes of the re- 

 spiratory, alimentary, and genito-urinary tracts. The interruption 



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