334 IJADIATION mOLOGY 



means of radiation to reach a virus within the host cell and to obtain 

 information as to its intracellular properties. 



Althou}i;h viruses do not constitute a homogeneous group that may 

 a piiori he expected to react to radiation in a uniform way, there does not 

 appear to be any fundamental dilTerence in the he^havior of different 

 \iruses toward ratliation, and most effects can he described under com- 

 mon headings for all viruses. The major differences reflect the different 

 purposes for which radiation experiments with different viruses have been 

 performed. In this chapter various types of radiation effects and appli- 

 cations of radiation analysis will be discussed, different viruses being used 

 as examples rather than each virus being followed separately through the 

 various approaches. It should be the goal of the radiobiologist to inter- 

 pret the effects of radiation on \'iruses in terms of chemical alterations in 

 nucleoproteins and other virus components. At this time, however, the 

 radiation chemistry of large biological molecules is so poorly understood 

 that a strictly chemical approach to the topic is precluded. 



2. EFFECT OF RADIATIONS ON VIRUS INFECTIVITY 



The most thoroughly investigated effect of radiation on viruses has been 

 the loss of infectivity or "inactivation " of free virus particles when 

 exposed to radiation. It must be recalled that "activity" of a virus can 

 be defined as the ability to reproduce and to cause a detectable manifes- 

 tation when introduced by a proper route into a suitable sensitive host. 

 Quantitative studies are made possible by the relative accura(;y of the 

 titration methods for viruses ; the amount of virus in different samples can 

 be compared fairly accurately with a precision that may vary from 5 per 

 cent in some instances to a factor of 2 or more in others. According to a 

 majority of authors the results of virus titration give values proportional 

 to the actual number of individual virus particles, each particle acting as 

 one infectious unit with a probability that may be much lower than unity 

 (Luria, 1940; Lauffer and Price, 1945). The results of titration can be 

 used, however, to compare the active virus content of different samples 

 (e.g., of an irradiated and a control suspension) even if infection requires 

 the summation of the action of large numbers of virus particles rather 

 than the reproduction of one successful particle, provided that a definite 

 rule of proportionality exists between the amount of active virus intro- 

 duced and the number or extent of the host manifestations. This is true, 

 for example, for a method of titration that uses the incubation period or 

 the time of death of an infected animal as related to the infecting dose 

 (Bryan and Beard, 1939; CJard, 1943) rather than the counting of indi- 

 vidual lesions or the dilution end point. 



One important caution for the radiobiologist is the control of the mode 

 of testing virus activity. This has become particularly important since 



