CHAPTEE VII. 



SPECIAL FEATURES OF INFECTION. 



(1) Virulence, Toxicity, Etc. 



Patiio- The word pathogenicity, in its relation to infec- 

 virnimce! tion, refers to the power of an organism to produce 

 disease, and often to the character of the changes 

 which it causes. Virulence in all essential respects 

 is synonymous with pathogenicity, but is used 

 more commonly in describing the degree of path- 

 ogenic power which a micro-organism posseses. 

 Thus virulent and a virulent (or non-virulent) 

 strains of the cholera vibrio or diphtheria bacillus 

 are spoken of. 



Toxicity. Toxicity refers to the poisonous properties of a 

 microbe or its secretions. As a property it is not 

 necessarily associated with the living micro-organ- 

 isms. The question is still discussed as to whether 

 toxicity and virulence are coextensive, even if they 

 are not identical properties. Undoubtedly, toxicity 

 is one of the factors on which virulence depends, 

 and, from the standpoint of the micro-organism, it 

 may be the sole property. Some organisms of little 

 or no pathogenic or infective power nevertheless 

 possess a protoplasm which is more or less poison- 

 ous, as certain aspergilli, penicillia or Bacillus 

 subtilis. 



infectivity. Infectivity, or infectiousness, relates to the 

 power of a micro-organism to maintain itself and 

 to multiply in a living host. One which is able to 



