1 82 THE STORY OF GERM LIFE. 



which either neutralize the poisons or act as 

 germicide upon the bacteria, or both, and perhaps 

 due in part to an active destruction of bacteria 

 by cellular activity (phagocytosis). There is little 

 reason to doubt that it is the same set of activi- 

 ties which renders the animal immune. The forces 

 which drive off the invading bacteria in one case 

 are still present to prevent a second attack of the 

 same species of bacterium. The length of time 

 during which these forces are active and sufficient 

 to cope with any new invaders determines the 

 length of time during which the immunity lasts. 

 Until, therefore, we can answer with more exact- 

 ness just how cure is brought about in case of 

 disease, we shall be unable to explain the method 

 of immunity. 



LIMITS OF PREVENTIVE MEDICINE. 



With all the advance in preventive medicine 

 we can not hope to avoid disease entirely. We 

 are discovering that the sources of disease are on 

 all sides of us, and so omnipresent that to avoid 

 them completely is impossible. If we were to 

 apply to our lives all the safeguards which bac- 

 teriology has taught us should be applied in order 

 to avoid the different diseases, we would surround 

 ourselves with conditions which would make life 

 intolerable. It would be oppressive enough for 

 us to eat no food except when it is hot, to drink 

 no water except when boiled, and to drink no 

 milk except after sterilization ; but these would 

 not satisfy the necessary conditions for avoiding 

 disease. To meet all dangers, we should handle 

 nothing which has not been sterilized, or should 

 follow the handling by immediately sterilizing the 



