MICROORGANISMS THE CAUSE OF DISEASES 281 
One of the most surprising developments along this line is the 
application of these principles to typhoid fever. In this case 
the organisms are not only weakened, but actually killed. The 
dead bacteria (killed at as low a temperature as possible) are 
injected under the skin and, strange to say, the person thus 
treated becomes immune to typhoid for some time. The exact 
explanation of this phenomenon is unknown, but it is thought 
that in getting rid of these dead bacteria the body in some way 
develops the power of destroying live typhoid organisms as well. 
The same principle has been applied to other diseases, but in no 
case with such entire success as with typhoid. 
MICROORGANISMS THE CAUSE OF DISEASES 
The studies of the last twenty-five years have demonstrated 
that the majority of diseases in animals and plants are produced 
by the growth of parasites of some kind, and mostly by what 
may be called microérganisms. Bacteria, yeasts, and molds 
are all concerned and, in addition, some diseases are produced by 
microscopic animals. The subject of germ diseases has become 
one of very wide range and cannot be considered to any great 
length in this work. The discussion of such diseases among ani- 
mals belongs to veterinary medicine, and of those among plants to 
botany. In our general consideration of microdrganisms as 
related to agriculture, we can review only the important prin- 
ciples involved, and give a brief survey of the more important 
diseases. 
