CHAPTER V 

 INVISIBLE MICROORGANISMS* 



The term "invisible microorganism" is used interchangeably with 

 such expressions as "ultra-microscopic organism," "invisible virus" 

 and "filterable virus" to designate a group of microorganisms which, 

 for the most part, 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,! 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 tj^ical 

 attacks of foot-and-mouth disease when inoculated into susceptible 

 animals. If now this infectious lymph be dUuted 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 

 wiU 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 Hving organism and not by a toxin, 

 because by successive filtrations and inoculations the disease can be 

 transmitted through a long series of animals, thus indicating clearly 

 that there exists in the filtered lyniph 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. 



t Bovine pleuropneumonia and such others as may respond to the cultural method of 

 Noguchi. 



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