400 BACTERIOLOGY. 



the effect upon the so-called constitutional dis- 

 eases like tuberculosis, which we must now 

 regard also as an infectious disease was 

 less marked and the real contagious diseases 

 like small-pox and scarlet-fever were only in- 

 directly affected. 



If we bear in mind what was remarked ear- 

 lier regarding the development of strict para- 

 sitism out of occasional parasitism (p. 257) 

 and that of this latter condition out of simple 

 putrefactive power, we are at once in a position 

 to understand how it is that works of sanitation 

 which directly improve the soil and water may 

 affect directly only those disease germs which 

 stand in some sort of relation to the outer 

 world. Measures of this kind cannot in general 

 act directly upon the occasional saprophytes, 

 and the strict parasites of the kind found in 

 tuberculosis and relapsing fever, and which 

 may possibly be present in the acute exanthe- 

 mata like typhus, small-pox and scarlet fever. 

 When such measures do exert an ameliorat- 

 ing effect upon these diseases and diminish 

 the number of cases, only an indirect action 

 can be claimed. This indirect action may be 

 exerted in different ways. These measures 

 may for example hinder or prevent the passing 

 over or transfer of disease germs to human 

 beings, and they may also, through the im- 



