IS THE PHYSICS OF VIRUSES 



Viruses 1950), and he concludes that //, the number of lesions, 

 is given by 



y - A^(l - e~ '"'■') 



where A' is the number of points at which infection could occur, 

 pv is the average number of virus aggregates which enter the 

 possible infection points, and x is the virus concentration. Thus, 

 in practice the procedure involves plotting an experimental 

 curve of y versus x on the same test })lants, and using this to 

 determine an unknown, .r. 



It is always assumed, and probably justifiably so, that a 

 single virus can cause infection. 



Lesion counting is possible for animal viruses, the chorio- 

 allantoic membrane of a chick embryo being suitable for use. 



Where any virus is characteristically recognizable in the 

 electron microscope, and where high enough concentrations are 

 available, this direct method of assay can be used. 



Origin of Viruses 



Here we are in deep waters. In the first place, it is not yet 

 the part of the physicist to describe the origin of a living thing, 

 or even something related to it. In the second place, there is no 

 general knowledge bearing on the origin of viruses. In the one 

 case of bacterial viruses, which may not be typical at all, the 

 phenomenon of lysogenesis, discovered by Listome and Carrere, 

 Bordet, and Burnet, seems to push one stage back toward the 

 origin of viruses. Certain bacteria, notably B. megaterium, carry 

 a pro-virus, which is manufactured at each cell division and 

 carried by each bacterium. Under the influence of ultraviolet 

 light, or of certain reducing agents, the pro-virus develops into 

 active virus, which then causes bacterial lysis, whence the 

 name lysogenic. The pro-virus is presumably an active, useful 

 part of the bacterial cell, which carries the capability of change 

 into a dominant destructive object. Studies of the pro-virus 

 are progressing rapidly, and it is one of the most exciting 

 branches of virology at the moment. 



The imjilication of the pro-virus is that all viruses originate 

 in host cells, either of the final host or one like it. We can, there- 



