18 
Another aspect of variation in size, of the 
human infectious dose depends upon 
individual differences in resistance to disease (Tables 10 and 11). This was 
demonstrated experimentally during studies on typhoid fever in man, in which 5% of 
the men resisted an oral challenge of 10' bacilli while the more susceptible 28% 
were made ill by ingesting 10" (253). Another example is available from the 
responses of chickens in the same inbred line highly susceptible to avian leukosis 
(188,215). Since humans are finally variable, then this gives impetus to the 
search for means to identify the constitutionally susceptible laboratory worker in 
viral oncology, 
TABLE 10. HUMAN ORAL DOSE PRODUCING TYPHOID FEVER 
Dose of Via ble JL Typhosa 
10 3 
10 5 
10 7 
10 ' 
Percen t Men with Typhoid 
None 
28% 
50% 
95% 
T ABLE 11. AVIA N LEUKOSIS VIRU S, DAI STRAIN k 
Viral Particles Inoculated Percen t Chicks Infecte d 
10 4 5% 
32 x 10 6 , 50% 
10 11 95% 
i 
i 
i 
[405] 
