CHAPTER IV. 

 INVISIBLE MICROORGANISMS* 



The term "invisible microorganism" is used interchangeably with such 

 expressions as " ultra- microscopic organism," " in visible virus" and 

 "filterable virus" to designate a group of microorganisms which cannot be 

 discerned with the most powerful lenses. Besides being invisible, these 

 microorganisms will pass through the ordinary "bacteria-proof" filters 

 and with one exception, f they have resisted all attempts at cultivation 

 outside of the animal body. 



The virus of foot-and-mouth disease may be taken as a typical 

 example. In this disease vesicles form in the mouths and on the feet 

 of infected cattle. The virus is known to be present in the lymph which 

 forms in these vesicles because this lymph will produce typical attacks 

 of foot-and-mouth disease when inoculated into susceptible animals. 

 If now this infectious lymph be diluted with water and passed through a 

 Berkefeld filter the resulting filtrate will be found to be free from all 

 visible microorganisms and in addition the usual culture tests will give 

 negative results. Notwithstanding this apparent sterility, however, the 

 filtrate will produce disease in cattle in the same manner as the unfiltered 

 lymph. It is known that the symptoms produced by the filtrate are 

 caused by a living organism and not by a toxin, because by successive 

 filtration s and inoculations the disease can be transmitted through a long 

 series of animals, thus indicating clearly that there exists in the filtered 

 lymph a living organism which is capable of reproduction. Another proof 

 that the virulence of the filtered lymph is caused by the presence of living 

 corpuscular elements, and that it is not a mere solution of a toxin, is 

 found in the failure of the virus to pass through filters of finer grain than 

 the Berkefeld as, for example, the Kitasato filter. 



The more important of the diseases which may be caused by invisible 

 microorganisms are yellow fever, infantile paralysis, hog cholera, bovine 



* Prepared by M. Dorset. , 

 fBovine pleuro-pneumonia. 



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