49 



VIRUSES AS ORGANISMS 



-^ \ \ \ \ 1^-^^ 



-3-2-1 I 2 3 



STANDARD DEVIATION UNITS 



FIGURE 38 - NORMAL CURVE 



If thi3 type of distribution of susceptibility within the host actually occurs, 

 then certainly one would expect to find that if the dosage level is low enough, 

 virtually no host individuals will become infected. On the other hand, if the 

 dosage level is somewhat higher then only the more susceptible of the hosts will 

 become infected, and as the dosage level becomes greater and greater, more and 

 more of the host individuals will become infected. Finally if the dosage level 

 is great enough, all of the hosts will become involved, even the least suscepti- 

 ble. If one constructs from a distribution of this sort a theoretical relation- 

 ship between the response in the host and the dosage level, one obtains the 

 broken curve in Figure 37* '^'he specific curve constructed was obtained for a 

 distribution of host susceptibility in which the standard deviation of the log- 

 arithm of susceptibilities was equal to exactly 1/2 logarithmic unit. This 

 theory of host-virus interaction has been advocated most strongly by Bryan and 

 Beard. Its essential feature is that the host has some mechanism for combat- 

 ing the virus. The more strongly developed that mechanism is within an indi- 

 vidual host, the greater the quantity of virus the individual can combat. If 

 the dose of virus does not exceed the ability of the host to combat, then the 

 host does not become infected. On the other hand, if the dosage is so great 

 that the defense mechanism becomes exhausted, then the individual succumbs. 

 Hosts vary in the extent of their defense mechanism. This theory is widely dif- 

 ferent from that outlined previously and yet it leads to an end prediction of 

 the variation of response with dosage which differs only slightly from that of 

 the previous theory. #e will now try to examine some experimental data to de- 

 termine whether or not it is possible to differentiate between these two view 

 points . 



Perhaps the best test of the theoretical deductions concerning the nature 

 of the dose response to dose level is afforded by the study of Parker with the 

 Board of Health strain of vaccinia virus. This strain is highly virulent for 

 rabbits Inoculated intradermally . Parker Injected quarter-cc portions of virus 

 suspension at various concentration into the rabbit skin. Severe necrotic les- 

 ions were formed. The results are presented graphically in iTigure 39» where 

 the fraction of inoculated sites showing lesions is plotted against the loga- 

 rithm of virus concentration. 



