VIII 



Viruses — Cell Parasites 



A good many years ago it was discovered that there are agents of 

 disease which are much smaller than bacteria, since they pass through 

 filters that retain bacteria and also cannot be seen under the micro- 

 scope. As long ago as 1892, Ivanowski observed that the sap of 

 tobacco plants which were infected with tobacco mosaic disease 

 still had the power of infecting healthy plants, when rubbed on their 

 leaves, even after it had been passed through a fine porcelain filter. 

 In 1898, Loeffler and Frosch showed that the infective agent of foot 

 and mouth disease of cattle could also pass through such filters. 



The importance of these observations was not at first understood, 

 but gradually a distinction was drawn between bacteria, which were 

 usually visible under the microscope and were stopped by fine filters, 

 and filter-passing agents which were submicroscopic and were called 

 viruses. 



The list of virus diseases, even of man, is now very long and includes 

 measles, mumps, influenza, poliomyelitis, chickenpox, rabies and 

 yellow fever. 



For many years viruses remained in the realm of the invisible. 

 Estimates of their size were made by various means. For example, 

 Dr W. J. Elford made collodion membranes having pores of different 

 sizes and was able to estimate the sizes of the different agents by their 

 ability to pass through the pores. It was found that, although sub- 

 microscopic, they were much larger than most protein molecules (see 

 Fig. 15). 



Methods of concentrating and purifying the virus material were 

 also worked out. Dr W. M. Stanley, in 1935, applied to infected 

 tobacco plant juice the methods of salt precipitation which had been 

 successfully applied to the separation of the enzymes. He obtained 

 a protein of high molecular weight, which was highly infective and 

 could be prepared in a semi-crystalline condition. Here was a pure, 

 or comparatively pure chemical substance, which had the abiUty of 



V 



