CONDITIONS MODIFYING PATHOGENICITY 183 



if a larger dose be 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, there- 

 fore, 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. Staphylo- 

 cocci 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 

 inflammatory reaction with subsequent destruction of the 

 organisms may be restricted to the site of infection or may 

 occur also in the related lymphatic glands. 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 intravenous injection. 



In the case of syphilis, inoculation of monkeys is more 

 successful by scarification than by any other means. 



2. The Subject of Infection. Amongst healthy individuals 

 susceptibility and, in inverse ratio, resistance to a particular 

 microbe may vary according to (a) species, (6) race and in- 

 dividual 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, syphilis, etc., which under natural 

 conditions are peculiar to the human subject and can only be 

 transmitted to a few of the animals. And further, there are 

 others, such as cholera and typhoid, the typical lesions of which 

 cannot be experimentally reproduced in animals, 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 



