CONDITIONS MODIFYING PATHOGENICITY. 159 



one or two organisms introduced will produce a fatal disease, 

 e.g. the case of anthrax in white mice. The healthy peritoneum 

 of a rabbit can resist and destroy a considerable number of 

 pyogenic micrococci without any serious result, but 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, therefore, for a par- 

 ticular 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 organ- 

 isms 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 illus- 

 trated 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. 



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 indi- 

 vidual 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 sub- 



