In every healthy individual are found multitudes of germs of both the pathogenic and harmless varieties. We are constantly exposed to the influence of the former yet by no means all bodies into which pathogenic germs find entrance contract disease. This fact has caused much study and among pathologists and bacteriologists generally the conclusion has been reached that the development of colonies of micro-organisms sufficiently to produce disease depends upon what is known as “susceptibility” of the organism. There must be a latent weakness of which the micro-organisms take advantage. This amounts to the admission that the body contains the inherent property of successfully resisting all germ action. Indeed, the fundamental proposition of Serum-Therapy is that under stress of the presence of dilute germ infusions the body _does_ develop special chemicals which neutralize the germ poisons and kill the germs and which remain after the inoculation to guard against any further entrance of germs of the same kind and vulnerable to the same protective chemicals. This theory is sufficiently correct to have served as an unassailable basis for a most illogical procedure. The truth is that the auto-protective power of the body must be lower than normal and the germs must find a weakened area for development and multiplication before they can develop sufficiently to produce disease. Once they gain a foothold they tend to multiply with great rapidity and to develop alarming symptoms often leading to death. Only in a few instances does modern science believe that a pathogenic germ can successfully attack a healthy body, but is claimed that there are a few germs, such as the Klebs-Loeffler bacillus (diphtheria producer) and the bacillus of anthrax, which may find lodgment in any organism, healthy or unhealthy, to produce disease. Now, the susceptibility of the body to germ invasion requires explanation. Merely to say that one is susceptible and another is not leaves too wide a field of possibility for error. It is easy to reason from the fact that all persons are at some time exposed to contagious or infectious diseases while comparatively few contract them that some persons are vulnerable to certain diseases while others are not. It is plain that while a person may be susceptible to typhoid fever because he has a weakness in the intestines, he may be quite immune from pneumonia or tuberculosis or any other infectious or contagious disease. But why this difference? Let us look at the problem from another angle.