SPECIFICITY OF ORGANISMS. 27 



is the cause of hog cholera. Eecent experiments, 

 however,, have disclosed that the true virus is ultra- 

 microscopic and filterable, the bacillus being only 

 a more or less constant associate. It is conceivable 

 that in some cases the combined action of two 

 micro-organisms may be necessary to cause the 

 disease. The non-toxic products of the two may 

 synthesize to form a toxic substance (Hektoen.) 3. 

 Unstainability of the microbe. 4. Ultramicro- 

 scopic size. The organism of the peripneumonia of 

 cattle was cultivated by Nocard and Eoux by grow- 

 ing it in a closed collodion sac which was placed in 

 the peritoneal cavity of suitable animals, It is so 

 small that its form can not be made out, and 

 growth is recognized only by clouding of the 

 culture medium, and the increased virulence of the 

 latter for animals. 



Some valuable information has been obtained 

 by observing whether the infectious agents are 

 so small that they will pass through dense filters 

 of porcelain or infusorial earth. It has been found 

 that the viruses of foot and mouth disease, peri- 

 pneumonia, rinderpest, sheep-pox, chicken-typhus, 

 horse sickness, epithelioma contagiosum of fowls, 

 yellow fever, hydrophobia, and hog cholera are fil- 

 terable. This is determined by injecting the fil- 

 tered culture medium or serum into susceptible 

 animals. The viruses of smallpox, vaccinia, and 

 Rocky Mountain spotted fever are not filterable. 

 Inasmuch as scarlet fever, measles, chicken-pox 

 and typhus fever cannot be produced in animals, 

 the filterability of their viruses is not at present 

 susceptible to determination. 



There is a marked tendency in many diseases, 

 typhoid, cholera, malaria, etc., for characteristic 



