A person who has once had measles, smallpox or whooping-cough does not, as a rule, have the infection a second time. We call this immunity. On the other hand, there are infectious diseases, such as pneumonia and erysipelas, which leave behind a greater sensibility, which we call predisposition.
For half a century the medical world has worked on the mechanism of these phenomena. The great discoveries made in this field have been the result of our knowledge of the pathogenic micro-organism, together with that of the morphological elements and the soluble constituents of the blood and other organs. It is not surprising that the greatest attention has been paid to microscopic observations and animal experimentation, while the study of the clinical phenomena has been comparatively neglected.
THE CLINICAL OBSERVATION OF THE MECHANISM OF IMMUNITY
The question we put ourselves is: what occurs if we try to reinfect an