CONDITIONS MODIFYING PATHOGENICITY 177 



In. increasing the susceptibility of a given individual, con- 

 ditions of local or general diminished vitality play the most 

 important part. It has been experimentally, proved that 

 conditions such as exposure to cold, fatigue, starvation, etc., 

 all diminish the natural resistance to bacterial infection. Rats 

 naturally immune to glanders can be rendered susceptible by 

 being fed with phloridzin, which produces a sort of diabetes, 

 a large amount of sugar being excreted in the urine (Leo). 

 Also a local susceptibility may be produced by injuring or 

 ■ diminishing the vitality of a part. If, for example, previous to 

 an intravenous injection of staphylococci, the aortic cusps of a 

 rabbit be injured, the organisms may settle there and set up an 

 ulcerative endocarditis ; or if a bone be injured, they may pro- 

 duce suppuration at the part, whereas in ordinary circumstances 

 these lesions would not take place. 



Such facts, established by experiment (and many others might 

 be given), illustrate the important part which local or general 

 conditions of diminished vitality may play in the production of 

 disease in the human subject. This has long been known by 

 clinical observation. In normal conditions the blood and 

 tissues of the body, with the exception of the skin and 

 certain of the mucous surfaces, are bacterium-free, and if a 

 few organisms gain entrance, they are destroyed. But if the 

 vitality becomes lowered, their entrance becomes easier and the 

 possibility of their multiplying and producing disease greatly 

 increased. In this way the favouring part played by fatigue, 

 cold, etc., in the production of diseases, of which the direct cause 

 is a bacterium, may be understood. It is important to keep in 

 view in this connection that many of the inflammation-producing 

 and pyogenic organisms are normally present on the skin and 

 various mucous surfaces ; and also that during epidemics of a 

 disease, e.g., typhoid, cholera, meningitis, diphtheria, the patho- 

 genic organisms may be present on the mucous membranes of 

 healthy individuals — that is, may have gained access to the body 

 without producing the disease. The actionof a certain organism 

 may devitalise the tissues to such an extent as to pave the way 

 for the entrance of other bacteria ; we may mention the liability 

 of the occurrence of pneumonia, erysipelas, and various suppura- 

 tive conditions, in the course of or following infective fevers. 

 In some cases the specific organism may produce lesions through 

 which the other organisms gain entrance, e.g., in typhoid, 

 diphtheria, etc. A notable example of diminished resistance to 

 bacterial infection is seen in the case of diabetes ; tuberculosis 

 and infection with pyogenic organisms are prone to occur in this 

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