434 MICROBIOLOGY 



Probable etiology. Many opinions exist concerning the 

 etiology of these affections. Bacteria, protozoa and enzymes 

 have each been thought to be their cause. Although the specific 

 infectious agent is not known it has been found that whatever 

 it is, it resides in certain tissues or organs of the infected indi- 

 vidual from which at some time and by some means it is trans- 

 mitted to others. 



Reasons for failure to discover the cause. All of these 

 diseases have been carefully studied by the methods that have 

 been effective in revealing the cause of many of the infectious 

 maladies. Many other methods have also been tried. The 

 failure to find the cause may rest in a number of conditions 

 concerning the nature of the infective agent among which the 

 following may be mentioned : 



Their incultiv ability. It is possible that the cause of these 

 diseases can not be cultivated outside of the body of their host 

 or perhaps hosts. They are possibly obligatory parasites. 



Mixed or symbiotic infections. It has been thought that 

 possibly these maladies were the result of a mixed infection. 



U attainability. It has been thought that possibly the 

 causal factors could not be stained by known methods and 

 because of their refractive nature could not be detected in the 

 fresh state microscopically. 



Ultramicroscopic size. This w r as Pasteur's explanation for 

 not finding the cause of rabies. Helmholtz showed that on 

 account of dispersion of light, organisms less than 0.2 p in 

 size could not be rendered visible to the usual eye. Many of 

 the known microorganisms are so small that they are on the 

 border line between the .visible and invisible. 



Filtration experiments. Various forms of mineral filters 

 porcelain (Pasteur), infusorial earth (Berkefeld) have 

 been of much service in securing filtrates free from known 

 bacteria. This method has been brought into use with the 

 diseases of unknown etiology by which it has been found that 

 the virus, whatever it is, of certain diseases can be removed 



