THE PROBLEM OF VIRULENCE 27 



in the following chapter (see p. 37). The former, however, is easily 

 comprehended, in fact, is to be expected. For the small number of 

 bacteria which gain entrance to the tissues in spontaneous infection 

 is entirely inadequate in itself to produce symptoms. It is neces- 

 sary that multiplication shall take place until the bacteria have ac- 

 cumulated in number sufficient to cause noticeable physiological 

 disturbance. That the interval necessary for this must vary accord- 

 ing to the number of bacteria originally introduced, the virulence of 

 these, and the specific resistance of the patient goes without saying. 

 Von Pirquet and Schick have suggested also that the incubation 

 time may correspond roughly to the interval during which the sub- 

 ject is becoming "allergic" or hypersusceptible to the bacteria or 

 virus. This will be discussed at greater length in the chapter on 

 anaphylaxis. 46 



But within the limits of the variations introduced by these fac- 

 tors the incubation time of each infectious disease if spontaneously 

 acquired is sufficiently uniform to be characteristic. Thus the pri- 

 mary lesion in syphilis follows the inoculation after an interval of 

 two to three weeks, rabies follows inoculation with street virus after 

 ab9ut four to six weeks, the period being somewhat dependent on the 

 location of the bite ; typhoid fever takes about two weeks to develop ; 

 gonorrhea about five to seven days; small-pox about two weeks; 

 yellow fever three to five days; and scarlet fever and diphtheria 

 about two to six days. In general, it may be stated that within the 

 limits observed for each particular infection the shorter the incuba- 

 tion time the more severe is the infection. Thus if tetanus follows 

 inoculation with the tetanus bacillus within seven days the prognosis 

 is far more grave than when the incubation time has occupied two 

 or three weeks. And if localized and general symptoms follow rap- 

 idly (within twenty-four to forty-eight hours) after a streptococcus 

 infection it is likely that the process is a very severe and virulent 

 one. 



46 Von Pirquet u. Scbick. Wien. kl Woch., 16, 1903, pp. 758 and 1244. 



