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against the infectiousness of cholera, yellow fever, diph- 

 theria, scarlatina, etc. Indeed, since the death-roll of 

 tuberculosis is greater, year after year, than that of any 

 one or perhaps all of these diseases combined, the argu- 

 ment, if it had any sense at all, would tend to prove the 

 excessive infectiousness of tuberculosis. Such an argu- 

 ment ignores the unquestioned and familiar fact that we 

 are not all equally susceptible to any one of the infec- 

 tious diseases ; even the most malignant cholera or yel- 

 low fever attacks only a portion usually a decided 

 minority of the community. Explain it as we may, 

 there is a something which we may call predisposition, 

 by virtue of which only certain individuals yield to in- 

 fection by cholera or by tuberculosis ; and the fact is, 

 that the number susceptible to tuberculosis seems smaller 

 than to any one of several other infections. Compara- 

 tively few of us attain maturity without having had 

 measles, scarlet fever, and whooping-cough at least ; yet 

 six-sevenths of us complete our pilgrimage without ex- 

 hibiting evidences of tuberculosis. That this is not mere 

 accident is shown by experiment : guinea-pigs and rab- 

 bits rarely, dogs and cats usually fail to respond with 

 general tuberculosis to inoculation with tuberculous ma- 

 terial. Even the deadly anthrax usually fails to destroy 

 carnivorous animals, although the most virulent material 

 be introduced ; and it was long ago pointed out by 

 Chauveau, and often confirmed, that although sheep are 

 very susceptible to this disease, yet some sheep resist all 

 experimental attempts at inoculation, even when large 

 quantities of fresh anthrax material are injected into the 

 animal. Dogs enjoy in general immunity against infec- 

 tion by anthrax ; yet young dogs are often successfully 

 inoculated. Infection implies, therefore, not simply a 

 virus capable of propagation in an animal, but also an 

 animal capable of permitting such propagation. All vari- 

 ations of this relative adaptability may be exhibited be- 

 tween animals of the same species and a given virus. To 

 affirm, then, that a disease anthrax or tuberculosis, for 

 example is infectious is to assert that it can be com- 



