662 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMAtS 



substances come together, a toxic substance may be produced. This 

 condition of affairs might explain certain infectious diseases in which 

 microorganisms are known to occur, and in which they cannot be 

 directly connected with the disease as causative factors. For example, 

 the Strept. pyogenes very frequently occurs in both scarlet fever and 

 smallpox. It has been shown absolutely that this organism is not the 

 cause of these diseases, but there is a remote possibility that it may 

 act in the so-called associative relation with some other microorgknism_ 

 or virus, as mentioned above, and produce the typical symptoms of 

 these diseases. It has been recently stated that scarlet fever is 

 due to a filterable virus but there is every reason to believe that the 

 occurrence of the Strept. pyogenes materially changes the character of 

 the infection and makes it more severe. The associative relationship 

 of infectious organisms is probably not the logical explanation for all 

 infections of this character. It might be mentioned in this connection 

 that the view is held by sonie investigators that some of the infectious 

 diseases of unknown etiology are due to enzymes and that a so-called 

 autocatalysis explains the seeming reproduction in the body of the 

 viruses. This theory is, however, without substantial proof. 



The Disteibtjtion of Pathogenic Microbic Agents in Nature 



The causal microorganisms of most of our infectious diseases are 

 found principally in the bodies of diseased man and animals. There are 

 some exceptions to their being found only in the bodies of the diseased. 

 Notable examples are found among certain of the wild animals such as 

 the brush-buck, wildebeast and others which serve as resetvoirs for 

 the micrgorganisms of some of the most fatal of protozoal diseases. 

 These animals seem to be naturally immune. . Various insects which 

 are factors in the transmission of certain infectious diseases do not suffer 

 from these diseases in any form and are naturally immune. The most 

 common source, however, is the diseased animal or human body. 

 There is no doubt, for example, that the natural habitat of the Bact. 

 diphtheria is in the throat and nasal passages of persons suffering from 

 or convalescing from diphtheria. Occasionally these bacteria are also 

 found in the nasal passages and throats of persons who have never had 

 diphtheria. The B. typhosus of tj^hoid fever also has its natural abode 

 in the intestinal tract of persons suffering from or convalescing from the 

 fever. The same is true with the majority of the causal microorgan- 



