48 CLINICAL BACTERIOLOGY. 



a whole city. Such dissemination can take place only 

 through the air, the earth, or the water, which are com- 

 mon to all, and are capable of acting upon the entire popu- 

 lation of a city at the same time and in the same way. In 

 this connection the demonstration made by Koch in recent 

 epidemics of cholera is of especial significance namely, 

 that the cholera-dejections gain entrance into the sewage 

 through the first unreported cases and into the public 

 waterways through those living along the streams (sailors), 

 and from both of these sources frequently into the water- 

 supply. An entire community may thus be simultaneously 

 infected with the contaminated drinking-water, and in this 

 way a cholera-explosion takes place. 



In addition to these more special causes, social conditions 

 naturally retain their significance as general causes of epi- 

 demics. An impaired state of nutrition affecting entire classes 

 in a community, the absence of air and of light in dwell- 

 ings, the abuse of alcohol, etc., must naturally increase the 

 predisposition to infection in the same way as lack of clean- 

 liness and density of population multiply the possibility of* 

 contagion. The disease-germs are, further, the less readily 

 gotten rid of, and they proliferate the more freely the more 

 filth and refuse defile the surroundings of human habita- 

 tions. In this sense contamination of the soil acts as an im- 

 portant cause, and purification thereof through a proper 

 water-supply and drainage as a fruitful means of prophy- 

 laxis, of the infectious diseases. The fact, also, that the 

 warm season of the year favors the growth and the virulence 

 of the causative agents of disease, and, on the other hand, 

 through the numerous digestive derangements resulting in 

 consequence of the increased heat and the thirst, increases 

 the predisposition of the community, may be mentioned in 

 further explanation of the occurrence of epidemics. Thus, 

 a series of factors may be traced that shed light upon the 

 previously mysterious origin of pestilences. On the other 

 hand, it must be emphasized that there is yet much that is 

 obscure in the etiologic relations of epidemics, and that 

 further investigation is necessary to clear up these questions. 



The Heredity of Infectious Diseases. The transmis- 

 sion to the offspring of chronic disease existing in either 

 parent at the time of conception may be considered as direct 

 infection of the sperm-cell or the ovum. To what extent 

 this actually takes place will be considered in discussing the 



