CONDITIONS MODIFYING PATHOGENICITY. 141 



rapidly fatal septicaemia when its virulence is raised. 

 Virulence also has a relation to the animal employed, as 

 sometimes on being increased for one species of animal it 

 is diminished for another. For example, streptococci, on 

 being inoculated in series through a number of mice, 

 acquire increased virulence for these animals, but become 

 less virulent for rabbits. (Knorr.) 



The number of the organisms introduced, i.e. the dose 

 of the infecting agent, is another point of importance. The 

 healthy tissues can usually resist a certain number of 

 pathogenic organisms of given virulence, and it is only 

 in a few instances that 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 becoming inflamed, but if a larger dose be intro- 

 duced, inflammation or suppuration will follow. Again, a 

 certain quantity of a particular organism injected subcutane- 

 ously 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. 



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

 following a direct entrance into the blood stream. Staphy- 

 lococci 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 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 



