PREFACE. Vll 



&c., but in particular to my assistants, Dr. Deneke and Dr. 

 Praussnitz, for the help which they have given me in this 

 arduous work. 



The second idea which guided me in altering the former 

 edition has reference to the etiology of the infective diseases. 



As to the mode of spread of these diseases we have as yet 

 received explanations almost entirely from the experiences of 

 medical practice, and from statistical observations. The 

 number of definite results obtained in this way has, however, 

 been extremely small, and where a law such as the peculiar local 

 and seasonal distribution of typhoid and cholera epidemics is 

 recognised we are, with regard to the significance and expla- 

 nation of the phenomena, thrown back on a series of more or less 

 probable hypotheses. 



The bacteriological investigations of the last few years have 

 produced a complete change in this respect. By the discovery 

 of numerous disease germs, and of the methods of their pure 

 cultivation, it has become possible to study experimentally the 

 conditions of life of the infective organisms, their mode of life, 

 their relation to their surrounding, their transportability, and the 

 mode of their entrance into man, and thus to obtain information 

 as to the causes of the peculiar mode of spread of epidemic dis- 

 eases, in a manner incomparably quicker and more trustworthy 

 than by empirical and statistical methods. 



I have as far as possible in the present work made use of 

 these advances of the most recent scientific investigations, the 

 bearing of which is decidedly underrated, and have attempted 

 to explain the mode of spread of infective diseases, more espe- 

 cially of cholera, by reference to the facts ascertained by experi- 

 ment as to the characteristics of the causal agents. 



It must be evident that we have already gained in this 

 way, in the case of a number of infective diseases, a deeper 

 insight into their mode of spread and the prophylactic measures 

 necessary in combating them. In the case of many diseases we 

 do not as yet fully understand their mode of spread ; nevertheless 

 by the aid of the results of bacteriological investigations we can 



