VIRUSES 



50 



-1 

 log- 6c) + log (vn) 



FIGURE 39 - FRACTION OF VACCIITIA VIRUS INOCULATIONS THAT 

 DEVELOPED LESIONS PLOTTED AS A FUNCTION OF LOGiq VIRUS CON- 

 CENTRATION. PARKER'S DATA FOR BOARD OF HEALTH STRAIN. 

 CURVE IDENTICAL WITH SOLID LINE ON FIGURE 37. (M.A. Lauffer 

 and W.C. Price, Arch. Biochem. 8, 449 (1945) ). 



The smooth curve fitting the data is the graph of equation (1), derived from 

 probability theory on the assxunption that one virus particle is sufficient to 

 induce an infection. It can be seen that the data fit the curve beautifully. 

 Haldane used a statistical method to evaluate the closeness of the fit of these 

 data to the theoretical equation. His results indicate very satisfactory agree- 

 ment. If one attempted to fit these data to equations derived from the assumpt- 

 ion that a minimum of two virus particles is necessary for inducing an infection, 

 very bad agreement would be found. These data indicate conclusively that if in- 

 fection is the result of the requisite number of vaccinia virus particles being 

 found in a favorable site, then that requisite minimum number is one particle. 

 This is true in spite of the fact that studies carried out by Smadel, Rivers and 

 Pickels show that on the average, about four vaccinia virus particles must be 

 introduced into a rabbit skin in order to produce a lesion. We can interpret 

 this by saying that it is necessary to introduce four particles because the pro- 

 bability is only about 2^% that a particle introduced will have aji opportunity 

 to induce an infection. 



Data are also available from studies on plant viruses. In the case of the 

 plant viruses, one rubs a virus solution over the surface of a leaf. In due 

 course, necrotic local lesions are produced on the leaf. One then counts the 

 number of lesions present on the leaf and plots that number as a function of the 

 logarithm of virus concentration. Data for various strains of tobacco mosaic 

 virus on Nicotiana glutinosa leaves were obtained by Bald, by Caldwell, and by 

 Beale. When the number of lesions expressed as the fraction of the number pos- 

 sible is plotted against logarithm of concentration, the data shown in Figure 

 40 are obtained. 



