6o INFECTIOUS AND PARASITIC DISEASES. 



their virulence and made harmless. Undoubtedly 

 the limitation of infectious diseases, which otherwise 

 would be impossible of control, is brought about in this 

 manner. Nature's means are simple, yet all-pervading, 

 wherefore she accomplishes more than the combined 

 efforts of men. And her forces are effective, and always 

 at our disposal, so that it behooves us to learn her ways 

 and profitably make use of her bounty. The forces 

 which nature employs are chiefly light, drying, dilution, 

 cold, and symbiosis (association). 



Among the natural agencies destructive to 

 Light. bacteria, sun-light occupies the first place. 

 It is by far the greatest destroyer of germ- 

 life. Few microbes can live in direct sun-light many 

 hours, the pathogenies being especially susceptible to its 

 influence. Its destructive influence is exerted whether 

 the bacteria are in a dry state, or in a liquid. Moreover, 

 it acts upon them whether the solution is clear, or is as 

 foul as sewage. For example, only twelve hours' 

 exposure to direct sun-light is required to render sewage 

 free from bacteria, i.e., sterile. It also kills spores, if 

 the exposure is longer. 



Sun-light has the power of acting directly 



Dryness, upon bacteria in liquids, as has just been 



stated. But it has another action equally 



important which enhances its disinfecting powers to a 



point scarcely equaled by any gaseous disinfectant, 



namely, drying. With the exception of spores, com- 



