522 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



to this class. These diseases cannot be inoculated readily into ani- 

 mals and consequently the virus cannot be secured in sufficient quan- 

 tities for study. It should be borne in mind that a possible expla- 

 nation of some of these diseases of unknown cause may be found in the 

 proposition that two microorganisms may each produce non-toxic sub- 

 stances and that when these non-toxic substances come together, a toxic 

 substance may be produced. This condition of affairs may explain 

 certain infectious diseases in which microorganisms are known to occur, 

 and in which they cannot be connected with the disease as causative 

 factors. It may be mentioned in this connection that Strept. pyogenes 

 very frequently occurs in both scarlet fever and smallpox. It has been 

 shown 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 microorganisms, as mentioned above, and produce the 

 typical symptoms of these diseases. The associative relationship of 

 infectious organisms is probably not the logical explanation for infections 

 of this character. The view is held by some that some of the infec- 

 tious 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, wildebeest 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 hi any form and are naturally immune. The most common 

 source, however, is the diseased animal or human body. The natural 

 habitat of the Bad. diphtheria is in the throat and nasal passages of 

 persons suffering from diphtheria or who are convalescing from diph- 

 theria. Occasionally these bacteria are also found in the nasal pas- 

 sages and throats of persons who have never had diphtheria. The 

 B. lyphosus, of typhoid fever, also, has its natural abode in the in- 



