62 INFECTIOUS AND PARASITIC DISEASES. 



chapter. Here reference is only made to the fact that, 

 as the number of bacteria which get into the body plays 

 a highly important part in the production of disease, 

 adequate dilution of infectious agents may make them 

 innocuous. Hence one value of dilution is evident. 

 Another effect of dilution is to separate the individual 

 microbes, and thereby permit of closer and more 

 thorough contact with them of the natural destructive 

 agencies. In practical disinfection dilution of the 

 infectious material is for this reason always practised. 

 If the material is semi-solid it is diluted with a disin- 

 fectant in which it is soluble; or it is broken up, if not 

 soluble, to permit of closer contact of the disinfectant 

 with as much of the material as is possible. If liquids 

 are to be disinfected, either a liquid disinfectant is used, 

 or one that is soluble in the solution to be disinfected. 

 In any case, sufficient time (one hour for the disinfec- 

 tants in common use) is given for penetration. 



By symbiosis is understood the living 



( . ^- X together of different species of organisms 

 (association). . ^ . ^ ^ 



in the same medium. 

 It further implies that they are mutually helpful and 

 beneficial. Among bacteria in nature a struggle for 

 existence is constantly taking place, which in conjunc- 

 tion with other natural forces serves to keep their num- 

 bers within limits. Of the two classes of bacteria, 

 parasites and saprophytes, the saprophytes are the more 

 numerous and by far the hardier, so that when they are 



