662 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



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 microorganism 

 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 filtrable 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 some 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 DISTRIBUTION 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 reservoirs for 

 the microorganisms 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 Bad. 

 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 same is true of the M . intracellularis var. meningitidis, 

 of cerebro-spinal fever. The B. typhosus of typhoid fever also has its 

 natural abode in the intestinal tract of persons suffering from or con- 

 valescing from the fever. The same is true with the majority of the 



