262 BACTERIOLOGY 



Their existence has been suspected for a number of years 

 but it is only comparatively recently that sufficient became 

 known of them to justify our speaking confidently of them; 

 and even now little more than their etiological potentialities 

 and some of their physiological reactions can be considered. 



For a long while it has been a puzzle that such character- 

 istic contagious diseases as certain of the acute exanthemata 

 in man and a number of typical transmissible diseases in 

 animals should have eluded all efforts to discover their 

 causes. By the customary methods of bacteriological 

 analysis nothing of a positive character is learned and yet 

 by the introduction into susceptible animals of bits of tissue 

 from the diseased animal, or small quantities of blood or 

 tissue juices or even of filtrates of emulsions of such tissues 

 or juices, it is possible in a number of instances to reproduce 

 the disease. It is such evidence as this that serves as the 

 basis for the belief in the existence of invisible or elusive 

 viruses for a number of diseases of man and animals and 

 a few for plants. 



The existence of such viruses has been demonstrated 

 in smallpox vaccine, measles, typhus fever, dengue fever, 

 poliomyelitis, and trachoma, among the diseases of man 

 and in foot and mouth disease, contagious pleuro-pneu- 

 monia, sheep-pox, rabies, cattle plague, chicken sarcoma, 

 and distemper of dogs among those of animals, and in the 

 mosaic disease of the tobacco plant. Sometimes such fil- 

 trates when placed under special methods of cultivation 

 show evidence of multiplication by clouding of the media 

 but with no development of recognizable morphological 

 structures in a few instances such cultures have shown the 

 development of minute spiral forms of organisms. (See 

 Leptospira icteroides. 



Though little or nothing that is convincing can be said 



