W. M. STANLEY 



Pathologists and bacteriologists have labored long and arduously 

 with viruses. They have found that a given virus will reproduce only 

 within the cells of certain specific living hosts, and that, although some 

 viruses will reproduce within the cells of several different hosts, other 

 viruses will multiply only within the cells of one given host or sometimes 

 only within certain specialized cells of that host. They have shown 

 that the primary pathological changes produced in cells by viruses are 

 either proliferative or degenerative in character. In some virus dis- 

 eases, such as yellow fever, poliomyelitis, and tobacco necrosis, de- 

 generative changes predominate; but in many, as in smallpox and fowl 

 pox, both proliferation and necrosis occur. Still other virus diseases, 

 such as Rous chicken sarcoma, Shope rabbit papilloma, and tobacco 

 enation mosaic, are characterized by a rapid and unorganized cellular 

 proliferation. 



Long before the discovery of viruses, a means was recognized of 

 protecting man against the virus disease, smallpox. This was achieved 

 by vaccination with active virus, presumably altered by passage in an 

 unnatural host. However, it has only been in recent years that there 

 has come a full realization of the great benefits, both with respect to 

 methods of protection against virus diseases and to the study of the 

 viruses themselves, that can be achieved through the use of new virus 

 hosts. Yellow fever has been eliminated as a major health problem, 

 and much has been learned of the virus because the virus was taken 

 from man and grown in monkeys, in mice, and in chick embryos. 

 The possibility of a recurrence of the 1918 influenza epidemic, which 

 killed more people than have died from combat activities in World 

 Wars I plus II (to date of writing), has been reduced and perhaps 

 eliminated because of the production of a vaccine which was made 

 possible by the growth of this virus in the chick embryo. Truly re- 

 markable progress has already been made in the study of viruses 

 and in the prevention of virus diseases, and it is to be expected that 

 this progress will continue. Yet withal, the one fundamental and all- 

 important problem posed by the viruses — that of the mode of virus 

 reproduction — remains unsolved. For many years little hope for a 

 solution of this problem was held, for viruses were generally regarded 

 as living organisms and the nature of life was considered to be a hal- 

 lowed, insoluble secret. However, chemists recognized in the virus 

 activity of certain crystallizable nucleoproteins a type of biological 



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