INFECTION 33 



tion are open, the parasite most commonly invades through 

 one that may, therefore, be regarded as the most appropriate 

 for entrance; this channel furnishes the typical picture of. 

 the infection. 



Susceptibility of the Host. Susceptibility varies in 

 different species of animals, in different members of the same 

 species, in the same individual at different times, and in the 

 same individual to different organisms. 



Susceptibility may be natural, as in smallpox; acquired, as 

 from exposure to conditions which lower the vitality, such 

 as hunger, cold, intoxication, fatigue, inhalation of noxious 

 vapors, and traumatic shock. Inherited susceptibility also 

 occurs. The transmission of certain inherited character- 

 istics, as narrow chest, predisposes to infection of the lungs. 



Mixed infections are the result of two or more micro- 

 organisms successfully invading and intoxicating the host 

 at the same time. 



Local Effects of Bacteria. By mechanical obstruction 

 from rapid growth of the bacteria, thrombosis, with its con- 

 sequences, may occur. Destruction of a part of the cells of a 

 tissue with necrosis can arise from irritation, the bacteria 

 acting as a foreign body. 



General Effects. Bacteremia or septicemia occurs when 

 bacteria proliferate and enter the whole system, as when 

 anthrax and typhoid cause general disease. 



Toxemia. When the poisons become widely distributed, 

 though the bacteria remain few and localized, and never or 

 seldom enter the circulation, as diphtheria and tetanus. 



Pyemia, a form of bacteremia, in which secondary or 

 metastatic foci of suppuration occur throughout the body. 



Suppurative bacteria are those which give rise to inflam- 

 mation and suppuration locally at the point of entrance, and 

 secondarily through metastasis. Any organism may cause 

 suppuration, but certain ones are peculiarly inclined to give 

 rise to pus, and are known as pyogenic organisms. 



Specific Bacteria. Infective bacteria are, as a rule, 

 specific, the particular toxin having a specific action and caus- 

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