l6o RELATIONS OF BACTERIA TO DISEASE. 



ject, differences in susceptibility to a certain disease are found 

 amongst different races and also amongst individuals of the 

 same race, as is well seen in the case of tubercle and other 

 diseases. Age also plays an important part, young subjects 

 being more liable to certain diseases, e.g. to diphtheria. Further, 

 at different periods of life certain parts of the body are more 

 susceptible, for example, in early life, the bones and joints to 

 tubercular and acute suppurative affections. 



In increasing the susceptibility of a given individual, condi- 

 tionsof 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 can be 

 rendered susceptible to glanders by being fed with phloridzin, 

 which produces a sort of diabetes, a large amount of sugar being 

 excreted in the urine (Leo). Guinea-pigs may resist subcuta- 

 neous injection of a certain dose of the typhoid bacillus, but if 

 at the same time a steriHsed culture of the bacillus coli be 

 injected into the peritoneum, they quickly die of a general 

 infection. Also a local susceptibility may be produced by injur- 

 ing or diminishing the vitality of a part. If, for example, pre- 

 vious 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 produce suppuration at the part, whereas in ordinary cir- 

 cumstances these lesions would not take place. The action of 

 one species of bacterium is also often aided by the simultaneous 

 presence of other species. In this case the latter may act simply 

 as additional irritants which lessen the vitality of the tissues, but 

 in some cases their presence also appears to favour the develop- 

 ment of a higher degree of virulence of the former. 



These facts, estabHshed by experiment (and many others 

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

 general conditions of diminished vitality may play in the pro- 

 duction 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 cer- 

 tain 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 pos- 



