THE PREVENTION OF INFECTIOUS DISEASE. 401 



provement of sanitary conditions, diminish the 

 predisposition of man to disease and increase 

 his power of resistance. Hygienic measures 

 therefore do not aim necessarily at preserving 

 every life at all cost, but rather at favorably 

 modifying the constitution of man, and ele- 

 vating the general hygienic condition of 

 the race. Whereas Pettenkofer sees in the 

 " circumstances of time and place " and their 

 modification by works of sanitation, an in- 

 fluence that can be exerted only upon the 

 parasites as found outside of the body, and 

 Koch considers of moment only the influence 

 of these measures upon the transmission of the 

 parasites, it was proved by Hueppe in 1887 

 that the " local disposition" consists also and 

 perhaps chiefly in the influence of the local 

 surroundings upon the human beings living in 

 the place. We may now hold this opinion 

 also on the basis of bacteriological evidence. 

 In 1889 Hueppe and Wood discovered that 

 harmless bacteria found in a certain locality 

 could confer protection against specific dis- 

 eases, while on the other hand Metschnikoff 

 discovered in 1894 that the invasion of the 

 cholera bacteria is favored by the presence of 

 certain saprophytes. Such " indigenous ' 

 saprophytic microbes may act upon the pre- 

 disposition toward disease possessed by the 

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