CHAPTER X. 



THE FILTERABLE VIRUSES. 



This general term means that the virus of a disease can pass 

 through a porcelain filter and usually that it cannot be seen by the 

 microscope. It, however, does not mean that it is invisible at all 

 stages since in one case at least we have been able by means of the 

 ultramicroscope to see what is almost certainly the particular causal 

 agent. Again it is said the spirochaetes when young will traverse 

 porcelain filters. The term will cover in this chapter those diseases 

 of importance to man whose causal agents cannot be morphologic- 

 ally described, but whose characters are more or less well known. 

 The list of diseases caused by submicroscopic agents is as follows: 

 African horse sickness, swamp fever of horses, catarrhal fever of 

 sheep, yellow fever, Dengue, three-day fever, typhus fever, polio- 

 myelitis, rabies, variola, with its congeners vaccinia and animal 

 pox, hog cholera, foot and mouth disease, fowl plague, fowl diph- 

 theria, transplantable sarcoma and leukemia of fowls, cattle plague, 

 trachoma, pleuropneumonia of cattle, molluscum contagiosum, 

 measles, scarlet fever, guinea pigs epizootic and some diseases of 

 plants. As said above, only the diseases transmissible to human 

 beings are reviewed. 



Hydrophobia. This disease has long been considered to be an 

 infectious one, but the causal parasitic agent has never been discov- 

 ered. It is commonly found in dogs, cats, wolves, rabbits, etc., but 

 other domestic animals, and man may become infected. It is a 

 disease of the central nervous system, highly infectious, always 

 following a bite or other injury in which the skin is broken, and 

 in which lesion the virus may be deposited. Infection may be 

 caused by injecting emulsified infected nerve tissue (brain) into 



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