158 MICROBIOLOGY 



the most frequent artificial method of infection, i. e., sub- 

 cutaneous injection, the bacteria are taken up through the 

 lymph-stream, and in part are held back in the lymph-glands, 

 where their virulence is weakened and where they may be 

 killed; but if the organisms are strongly "pathogenic" they 

 resist total destruction, and soon begin to multiply. Joest 

 and his associates have found tubercle bacteria in a large 

 number of apparently healthy lymphatic glands. 



Infection. Infection takes place when microorganisms 

 can remain alive and multiply after they have been intro- 

 duced into the tissues, or when they become localized on the 

 surface of a mucous membrane, multiply and in so doing 

 elaborate a poisonous toxin which is absorbed and which acts 

 injuriously upon the host. 



Specific infection. This refers to the entrance in or upon 

 the body tissues of the organism of one of the specific diseases 

 such as anthrax, glanders or tuberculosis. The bacteria be- 

 longing to this group cause rather definite tissue reactions so 

 that the lesions produced by each of them are more or less 

 characteristic. This makes it possible when the disease takes 

 its usual form to predict with considerable certainty the 

 etiological factor. 



Wound infection. This term is applied to the entrance 

 of bacteria into wounds and their multiplication there. The 

 wounds may be either accidental or surgical in origin. Many 

 bacteria are able, when introduced into the tissues, to produce 

 local inflammation, leading to suppuration or exudates of dif- 

 ferent kinds. It is not ordinarily possible to predict from the 

 lesions the species of bacteria causing the disturbance. 



Cause of variation in the form of infectious diseases. 

 The infectious diseases vary in their manifestation with the 

 variation in the degree of resistance (immunity) of the in- 

 dividual affected or variation in the virulence of the infecting 

 organism. This condition may be expressed in the following 

 formula : x 



1 Smith and Moore, Bulletin No. 6, Bureau of Animal Ind., 1894. 



