G6 INTRODUCTION. 



logical chemical standpoint, they have heen termed 

 "organised ferments"; on the other hand, from the 

 special pathological point of view, " parasitic plants " 

 or "micro-parasites." 



The object of the present work is to describe the form 

 and vital characteristics of these micro-organisms in so 

 far as they have a direct or indirect interest from a 

 hygienic point of view. 



The method The plan of this work embraces in the first place a 

 n ' short historical sketch of the development of our know- 

 ledge with regard to the ferments and parasites during 

 the last few years. Then follows a description of the 

 form and mode of development of those micro-organisms 

 which are important from a hygienic standpoint, and also 

 a short morphology and classification, a knowledge of 

 which is indispensable for the comprehension and the 

 further successful study of these bodies, so difficult 

 to recognise and to distinguish one from the other. 

 Nor is the biology of the micro-organisms less instruc- 

 tive; hence the third part deals with the general 

 conditions of life of the lower fungi ; in the fourth are 

 described the results of their life, their tissue change, 

 and energy, as well as their action as exciting agents of 

 fermentation and of parasitic diseases; and the fifth 

 chapter deals with the conditions of decay of the micro- 

 organisms and with the means which cause their 

 enfeeblement or death. 



In the sixth and seventh divisions the points which 

 are of special interest in hygiene are referred to ; in the 

 sixth the distribution of the various micro-organisms in 

 our surroundings, in air, soil, water, food, and dwelling ; 

 and in the seventh there is given a digest of the 

 conclusions to which we must come with regard to 

 the etiology and prophylaxis of the infective diseases. 

 The contents of this chapter include the discussion of 

 the external sources of infection, of the local and 

 seasonal predisposition to infective diseases, of the 

 mode of entrance of the infective agents into the 

 organism and their fate in the body, of the means by 

 which it is possible to obtain protection against the 



