CHAPTER XXXVI 



INFECTIOUS DISEASES WHOSE CAUSAL ORGANISMS ABE 

 NOT CERTAINLY KNOWN 



A CONSIDERABLE number of diseases regarded as infectious 

 are known in which the causal organisms have not been 

 discovered or are not certainly known. It may be asked 

 why should a disease be termed infectious, that is, caused 

 by some organism, when the organism itself is not known? 

 How can one be sure that an organism is responsible for a 

 disease before such has been proved to be the cause? The 

 reasons may be summarized as follows: All the diseases 

 discussed are more or less contagious, that is, more or less, 

 readily transmitted from one individual to another. It is 

 evident that something must actually pass from one indi^ 

 vidual to the next. This something is termed a virus. In 

 many of the cases the cause originally described as a virus 

 has been discovered, studied and proved to be a specific 

 microorganism. It is assumed that the virus in all cases is 

 a microorganism of some kind. 



The reasons why the causal organisms have not been 

 isolated and described for all diseases listed below can 

 scarcely be determined in advance. Probably in some cases 

 the organism will not grow in any of the media which have 

 been tried; in other cases the microscopic examination of 

 the lesions of the disease show bodies, which may or may not 

 be microorganisms, but which have not been cultivated as 

 yet. In other cases the organism, at least at certain stages 

 in its life cycle, may be ultramicroscopic. It is possible that 



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