268 BACTERIOLOGY. 



a true internal cause is to be found, inherent 

 in the internal organization of man. Just as 

 in all natural processes without exception, so 

 here, the disease germs act as liberating im- 

 pulses and are able to set free only what in the 

 form of a predisposition toward disease is in 

 some way prefigured both in nature and amount 

 in the human body. 



The dependence of either resistance to or dis- 

 position to disease upon the conditions of life, 

 as well as a like dependence of the disease 

 germs upon their own conditions of existence 

 inasmuch as they likewise are living organisms 

 explains, without recourse to violent assump- 

 tions, such facts as that insignificant, local, in- 

 fectious diseases may become world-wide,as chol- 

 era has become in our own century, that new 

 infectious diseases may make their appearance, 

 as for instance cerebro-spinal meningitis in 

 the last hundred years, and that diseases once 

 widely spread like leprosy and the bubonic 

 plague may dwindle almost to the vanishing 

 point. We can easily understand the fact also 

 that, even under conditions originally very dif- 

 ferent, similar cultural influences arising from 

 similar unsanitary social conditions lead every- 

 where to the same danger from diseases such 

 as tuberculosis, for the reason that such condi- 



