142 RELATIONS OF BACTERIA TO DISEASE. 



injection of the pneumococcus produces death more readily 

 than intravenous injection. 



2. The Subject of Infection. Amongst healthy individ- 

 uals susceptibility and, in inverse ratio, resistance to a 

 particular microbe may vary according to (a) species, (ft) 

 race and individual peculiarities, (c) age. Different species 

 of the lower animals show the widest variation in this 

 respect, some being extremely susceptible, others highly 

 resistant. Then there are diseases, such as leprosy, gonor- 

 rhoea, etc., which appear to be peculiar to the human 

 subject and have not yet been transmitted to animals. 

 And further, there are others, such as cholera and typhoid, 

 which do not naturally affect animals, and the typical 

 lesions of which cannot be experimentally reproduced in 

 them, or appear only imperfectly, although pathogenic 

 effects follow inoculation with the organisms. In the case 

 of the human subject, 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, 

 conditions 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 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 subcutaneous in- 

 jection of a certain dose of the typhoid bacillus, but if at 

 the same time a sterilised 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 



