THE PREVENTION OF INFECTIOUS DISEASE. 403 



provisionally, though not quite precisely, the 

 number of sick persons in the community by 

 the actual mortality, then the total mortality can 

 be looked upon as an expression of the force of 

 constitution or of the sum of the bodily energy 

 of the inhabitants of a town or country, that 

 is convertible into effect not by a physio- 

 logical but by a pathological stimulus. The 

 contrast between physiological and patholo- 

 gical is of course employed only in an an- 

 thropocentric sense ; for nature herself deals 

 only with a struggle for existence, with a 

 selection of the fittest to resist disease. If we 

 remember furthermore that all forms of par- 

 asitism can develop and must have developed 

 out of the putrefactive processes so essential 

 in the cycle of life phenomena, we are at once 

 able to understand how, so long as there 

 exists available energy in a form departing 

 from that of the normal physiological or- 

 ganization, nature will cause an increase in 

 some diseases, a decrease in others, or permit 

 wholly new diseases to develop out of the 

 numberless possibilities which putrefactive 

 processes hold always in store. 



A real and permanent decrease in infec- 

 tious disease is possible only when there 

 exists less energy capable of release .in this 

 form, when in other words we remove the pre- 



