1 8 Insects and Disease 



to different conditions that makes many of the 

 kinds dangerous. The bacillus which causes 

 tetanus or lockjaw will illustrate this. It is a 

 rather common bacillus in soil in many localities. 

 As long as it remains there it is of no special im- 

 portance, but if it is introduced into the body 

 through a scratch or any other wound it becomes 

 a very serious matter. 



We may say, then, that the effect the bacillus has 

 on the host depends largely on the host. Not only 

 does it depend on what the host is, but the particu- 

 lar condition of the host at the time of infection 

 is of importance. Children are subject to many 

 diseases that adults seldom have. Hunger, thirst, 

 fatigue, exposure and other factors may make a 

 person susceptible to the actions of certain bac- 

 teria that would be harmless under other condi- 

 tions. 



The minute size and great numbers of the bac- 

 teria make their dissemination a comparatively 

 simple matter. They may be carried in the air 

 as minute particles of dust; they may be carried 

 in water or milk; they may be carried on the cloth- 

 ing or on the person from one host to another, or 

 they may be disseminated in scores of other ways. 

 In other chapters, particularly the one dealing with 

 the house-fly and typhoid, we shall see how it is 



