94 THE STORY OF THE BACTERIA. 



species of bacteria belonging to each must be 

 present, predisposition or no predisposition, or 

 the disease cannot occur. 



Typhoid fever and cholera are often called 

 filth diseases, and to bad food, foul air, sewer 

 gas, and overcrowding their occurrence has 

 often been attributed. This is in a sense true, 

 since these adverse conditions are apt to induce 

 a state of the body which renders it less resist- 

 ent than it should naturally be to various dele- 

 terious agencies ; but no imaginable degree of 

 unsanitary conditions could ever induce tuber- 

 culosis, or typhoid fever, or Asiatic cholera 

 without the presence of the particular germ 

 which causes each. None of these diseases 

 can spring up among any class or condition of 

 people without the introduction of the germ 

 from outside. 



The recently acquired knowledge of the 

 cause of Asiatic cholera has thus far aided but 

 little in the treatment of persons already its 

 victims. On the other hand, knowing defi- 

 nitely, as we now do, what causes the disease, 

 how and under what conditions it spreads, and 

 what will destroy the germs, we are to-day in 



