244 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 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, yellow fever, measles, typhoid fever, dengue 

 fever, poliomyelitis, and trachoma, among the diseases of 

 man and in foot and mouth disease, contagious pleuro- 

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

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

 mosaic disease of the tobacco plant. 



Though little or nothing that is convincing can be said 

 of the morphology of this group of ultra-microscopic par- 

 ticles, still in their reactions to a variety of physical agents 

 they are obviously living matter, having many analogies to 

 the more highly developed microorganisms with which we 

 are familiar. Practically all are killed at temperatures 



