CONDITIONS MODIFYING PATHOGENICITY 151 



introduced, a fatal peritonitis may follow. Again, a certain 

 quantity of a particular organism injected subcutaneously may 

 produce only a local inflammatory change, but in the case of a 

 larger dose the organisms may gain entrance to the blood stream 

 and produce septicaemia. There is, therefore, for a particular 

 animal, a minimum lethal dose which can be determined by 

 experiment only ; a dose, moreover, which is modified by various 

 circumstances difficult to control. 



The path of infection may alter the result, serious effects often 

 following a direct entrance into the blood stream. Staphylococci 

 injected subcutaneously in a rabbit may produce only a local 

 abscess, whilst on intravenous injection multiple abscesses in , 

 certain organs may result and death may follow. Local inflam- 

 matory reaction with subsequent destruction of the organisms 

 may be restricted to the site of infection or may occur also in 

 the lymphatic glands in relation. The latter therefore act as a 

 second barrier of defence, or as a filtering mechanism which aids 

 in protecting against blood infection. This is well illustrated in 

 the case of " poisoned wounds." In some other cases, however, 

 the organisms are very rapidly destroyed in the blood stream, 

 and Klemperer has found that in the dog, subcutaneous injection 

 of the pneumococcus produces death more readily than intra- 

 venous injection. 



2. The Subject of Infection. Amongst healthy individuals 

 susceptibility and, in inverse ratio, resistance to a particular 

 microbe may vary according to (a) species, (b) 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, gonorrhoea, 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 suppura- 

 tive affections. 



