CHAPTER XXVIII 



DISEASES CAUSED BY FILTRABLE VIRUSES. DIS- 

 EASES OF UNKNOWN ETIOLOGY 



A NUMBER of infectious diseases are caused by organisms that 

 are able to pass through a fine porcelain filter. That the causal 

 agents are present in the filtrate has been proven by animal in- 

 oculations ; yet in a certain number of cases the highest degree of 

 magnification possible fails to reveal their presence. Such organ- 

 isms evidently are either " ultramicroscopic " or they cannot 

 be rendered visible by present methods. Just what determines 

 the ability of an organism to pass through a filter is uncertain ; 

 plasticity may be as important a factor as minuteness. A few 

 of the filtrable viruses, although exceedingly minute, are still 

 within the range of visibility. 



The groups apparently bear no close relationship one to the 

 other. Their modes of transmission are also widely different. Cer- 

 tain forms, for example, are transmitted by biting insects, as in 

 yellow fever ; others by direct contact through a wound or abrasion, 

 as in rabies ; still others by contact, as in cattle plague. Eventually, 

 certain members will in all probability be classified with the pro- 

 tozoa and others with the bacteria. 



In order to be certain that an organism is filtrable several pre- 

 cautions must be observed in carrying out the filtration process. 

 The integrity of the filter must first be tested with a culture of 

 known infiltrable organisms, all of which must be retained and 

 not pass into the filtrate ; after which the filter must be sterilized 

 to insure its freedom from all germs. The pressure or suction 

 should only be moderate ; and the filtration should be completed 

 within two hours ; otherwise certain bacteria might grow through 



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