THE CAUSE OF INFECTIOUS DISEASE. 259 



parasites which act especially through the for- 

 mation of local growths or tumors. To the 

 latter class belong the germs causing tumors, 

 which have been investigated with especial 

 accuracy among plants, and a well-known ex- 

 ample of which is the germ of human tuber- 

 culosis ; to the former belong the germs of 

 diphtheria and tetanus ; the cholera germ also 

 approximates to the former group. Between 

 these extremes stand the other pathogenic bac- 

 teria : in some the proliferation of the bacteria, 

 in others their production of poison is the pre- 

 dominant factor. Disease-producing bacteria 

 may therefore affect man in very different 

 ways. They may cause changes by growing 

 and multiplying in vital organs and through 

 thus altering the metabolism of important tis- 

 sues may influence unfavorably the metabol- 

 ism of the whole body ; or they may rob the 

 body of important nutrient material and intro- 

 duce the products of their own metabolism 

 into the body of their host : or they may, in 

 the act of satisfying their own need of energy, 

 split off from the proteids of the human body 

 certain substances which act upon man as poi- 

 sons ; or they may themselves generate poisons 

 in their own bodies and, like poisonous plants, 

 be in themselves poisonous. The mode of 

 action may vary according to conditions ; for 



