CHAPTER I 



VIRUSES AS MOLECULES 



Shortly after the middle of the last century, Louis Pasteur succeeded in 

 demonstrating that diseases could be caused by specific pathogenic bacteria. 

 This was a startling discovery, one which inspired scholars the world over to 

 seek for bacteria as the causative agents of disease. During the thirty years 

 that followed Pasteur's demonstration, a great many of the common maladies of 

 man and other animals and even plaints were shown definitely to be caused by 

 specific microorganisms - living bodies which could be cultured artificially 

 and which were large enough to be seen with a good microscope and to be re- 

 tained by certain types of filters. In view of this background, it can be 

 appreciated that the Russian, Iwanowski, must have been exceedingly puzzled 

 when he discovered, in I892, that if he passed the juice from tobacco plants 

 diseased with tobacco mosaic through one of these filters which would hold 

 back all of the living organisms known at that time, the infectious principle 

 was not held back, but passed through with the fluids. This observation sim- 

 ply did not fit in with the spirit of the times, and Iwanowski ignored his 

 finding. Six years later a Dutchman named Beijerinck, having confirmed this 

 observation, realized that the infectious principle of tobacco mosaic differed 

 from ordinary bacteria and described it as a "contagiiim vivum fluidiun". Thus, 

 the vast field of filterable virus research was opened just before the begin- 

 ning of the present century through the study of tobacco mosaic - a disease 

 destined to become of unusual scientific interest. 



Since tobacco mosaic virus will be the subject of considerable subse- 

 quent discussion, it might be well to exiamine the symptoms of a tobacco plant 

 diseased with tobacco mosaic. A healthy tobacco plant has dark green, sym- 

 metrical, smooth leaves. A diseased plant is stunted and has leaves which 

 are unsymmetrical, mottled and distorted. Jf'igure 1 is a picture of a leaf 

 taken from a tobacco plant diseased with tobacco mosaic. 





