Preventable Diseases

The treaty between these germs and the body, however, is only of the "most-favored-nation" class; for let these tamed and harmless friends of the family escape and enter the body of another human being, and they will attack it as virulently as ever.

Now, where and how did nature ever succeed in getting the rehearsal and the practice necessary to build up such an extraordinary and complicated system of defense as this? Take your microscope and look at a drop of fluid from the mouth, the gums, the throat, the stomach, the bowels, and you will find it simply swarming with bacteria, bacilli, and cocci, each spe