4 5 VIRUSES AS ORGANISMS 



minutes. The 10 billion fold increase in tobacco necrosis virus mentioned pre- 

 viously, which required only sixteen hours, could be achieved by as few as 3^^ 

 successive divisions of the virus particles. This would represent just slightly 

 more than two divisions per hour. It is also possible to conceive of virus mul- 

 tiplication in terms of an auto-catalytic reaction. It is known for example, 

 that pepsin can be produced from a precursor pepsinogen. This reaction is cata- 

 lyzed by pepsin itself. In other words when purified pepsinogen is inoculated 

 with a trace of the enzyme pepsin, the entire mass of pepsinogen is gradually 

 changed over into pepsin. This is an auto-catalytic reaction. The growth of 

 viruses in the tissues of a living host could also be a reaction of this sort. 

 One could postulate that a single virus particle or molecule introduced into the 

 cell of a living host can act as a specific catalyst for the synthesis of simi- 

 lar virus molecules present in the host. In this case, the precursors would 

 have to be fairly simple materials - perhaps even amino acids. V^hich of these 

 two alternative views of virus multiplication represents reality cannot at pres- 

 ent be stated with any degree of assurance. 



Viruses have other properties in common with living organisms. Viruses 

 are able to undergo mutation. Even before viruses were recognized as entities, 

 it was known that strains could exist. The practice of vaccination against 

 small pox by infecting with a strain causing a related but milder disease dates 

 back to the latter part of the l8th century. Pasteur, in his memorable experi- 

 ments on rabies, passed rabies virus from generation to generation in the braina 

 of rabbits. At the end of many generations, he obtained an agent which produced 

 a disease very mild compared with the original, but which nevertheless could con- 

 fer immunity to rabies. This demonstration that a virus can be transformed into 

 a less virulent strain by laboratory manipulations also occurred before viruses 

 were recognized as specific entities. Today it is known that many viruses can 

 be modified by passing them through unnatural hosts. An Interesting example can 

 be had from the case of influenza virus. As was discussed previously. Influenza 

 virus has the ability to precipitate or agglutinate red blood cells of various 

 animal species. This reaction can be made quantitative. Burnet has shown that 

 influenza virus obtained from mouse lungs precipitates the red blood cells of 

 mammals better than the red blood cells of birds, that is, the quantity of virus 

 required to precipitate a fixed quantity of blood is less for mammalian blood 

 than for avian blood. On the other hand, when this virus is cultivated in the 

 chicken embryo for only a few generations, then it precipitates red blood cells 

 of birds better than red blood cells of mammals. Many other instances of the 

 modification of a virus by passage through an unnatural host could be mentioned. 

 The production of a mild strain of dengue virus by passage through mice, re- 

 cently announced by Sabin and Schlesinger, and of a non-virulent strain of yellow 

 fever virus by passage through tissue culture as accomplished by Theller, are 

 examples. In both of these cases the modified virus can be used as a vaccine to 

 immunize the Individual against the more virulent disease. 



It is believed by some that the modification of a virus by passage through 

 an unnatural host is a process of selection of a strain adapted to the new host. 

 It is believed, further, that viruses mutate frequently. When the virus is 

 transferred to a new host, sooner or later a strain of the virus better adapted 

 to the new host will arise by mutation. After that, this new strain will be 

 propagated preferentially by the unnatural host. Mutations are known to occur 

 in plant viruses as well as in animal viruses. Carsner observed that when the 

 virus which causes the disease curley-top of sugar beets was passed through the 

 strange plant, Chenopodium murale, a strain of the virus was selected which pro- 

 duced a much milder disease when taken back to the sugar beet. Johnson found 

 that tobacco mosaic could be altered by heating. Holmes extended this observation 

 and actually produced a strain of tobacco mosaic virus symptomless in tobacco by 

 the simple expedient of heating plant tissues infected with the virus. KcKlnney 

 was the first to study the bright yellow spots which occasionally developed on 

 leaves of tobacco plants diseased with tobacco mosaic. An illustration of such 



