CHEMICAL STUDIES ON VIRUSES — STANLEY 359 



tious agent, something we now call a virus, and was willing to believe 

 that the infectious agent was merely comprised of bacteria which had 

 passed through the holes in the filter. This was his unfortunate con- 

 clusion despite the fact that he had tested the filter many times against 

 known bacteria and the filter had always retained such bacteria and 

 despite the fact that he could find no bacteria in the filtrate. Recogni- 

 tion of the discovery of viruses was made quite independently just 6 

 years later by the Dutch botanist Beijerinck, who carried out similar 

 experiments with the same disease. He passed the juice from macer- 

 ated tobacco mosaic diseased plants through a bacteria-proof filter and 

 showed that the filtrate would cause the disease when applied to normal 

 plants. In addition he eliminated the possibility of the effect being 

 caused by a toxin by showing that the juice from these filtrate-inocu- 

 lated plants would still cause the disease after being passed through a 

 second filter, and so on in a series. Beijerinck announced that he had 

 discovered a new type of infectious agent, which he designated as a 

 "contagious living fluid." If you analyze this description you will 

 recognize that Beijerinck was not thinking in terms of bacteria or any- 

 thing like bacteria, but of something new and different. The use of the 

 words "living" and "fluid" is most significant, the latter especially so 

 to chemists. 



The same year Loeffler and Frosch discovered that the foot-and- 

 mouth disease of cattle was caused by a similar filterable agent, and in 

 1901 Reed and coworkers made the first recognition of a virus disease 

 of man, that of yellow fever. Since that time more than 300 different 

 diseases of animals, man, plants, and even bacteria have been found to 

 be caused by viruses. Among the virus-induced diseases of man are 

 smallpox, poliomyelitis, measles, mumps, chicken pox, virus pneumo- 

 nia, certain types of encephalitis, influenza, fever blisters, and probably 

 the common cold. Virus diseases of animals include hog cholera, cattle 

 plague, rabies, f owlpox, Newcastle disease of chickens, distemper, and 

 certain benign as well as cancerous growths. Among the plant virus 

 diseases are aster yellows, alfalfa mosaic, curly top of sugar beets, 

 tomato bushy stunt, quick decline of citrus, tulip break, potato yellow 

 dwarf, and peach yellows. 



The earliest recognized property of the agents causing these diseases 

 that was used to differentiate them from bacteria, namely, their filter- 

 ability, has come to be recognized as untenable, for some of these 

 viruses will not pass filters that will permit known organisms to pass. 

 However, it has been replaced by certain other properties that are 

 regarded today as characteristic of viruses. These emphasize the inti- 

 mate relationship that exists between viruses and host cells, the fact 

 that no virus has been grown on cell-free media, the fact that during 

 growth or reproduction viruses can mutate, the fact that most but not 



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