of the physical sciences could not, have yielded a single concrete fact, that one method — vaccination — and the most perfect one yet discovered of preventing a disease, and two drugs — quinine and mercury — specific for two other infectious diseases, should have been found and so successfully applied. But in contrast to this slow, painful and halting advance in practical means for the relief of suffering, is to be placed the body of robust facts, acquired in a quarter of a century, during the present or bacteriological era in medicine, which enables us to view in some measure the mechanisms of disease and defense against it, and which has pointed the way to efficient modes of prevention, and, in a few brilliant instances, to the production of biologically perfect means of combating certain infectious maladies. To produce a means, as has been done through the perfection of curative sera, that shall strike down myriads of living parasitic organisms, within the interior of the body, amid millions of sensitive and even sentient cells of the organs, without inflicting on them the smallest injury, is indeed a great accomplishment. And if I am successful to-day in placing before you the main facts, now revealed, of the body's manner of defense to parasitic invasion, you will, I think, come to see that it has been by imitating nature's methods and by augmentation of the natural forces of defense, that good has been achieved.
The facts laboriously acquired, on which this presentation will rest, have been drawn from the study of spontaneous disease —so-called natural disease — among man and animals, and from experimental diseases produced in animals. I need scarcely point out that there is really no unnatural form of disease any more than there is a really natural one; in all instances we are dealing with natural laws of health and disease, the difference merely being that in one case we are often ignorant of the time and manner of entrance of the infecting germs into the body, and in the other they are purposely introduced, in a predetermined efficient manner, in a pure state into the animal body. Since we are so often ignorant of the precise manner of ingress of the germs in the non-experimental forms of disease, we conclude from the identity of the conditions present in the experimental and non-experimental forms of the disease, that in effect they are identical. This power exactly to reproduce at will, by pure bacterial cultures, infectious disease in animals has been of inestimable benefit in investigating disease.To escape disease is not merely to remain without the zone of influence of the germs of disease. To do this in all cases is impossible, because with certain germ diseases — tuberculosis, for example — the germs are ubiquitous; and with several other diseases the germs are constant if not naturalized inhabitants of the body. Thus we carry on our skin surfaces constantly the germs of suppuration; on the mucous membranes of the nose and throat the germs of pneumonia, and sometimes those of diphtheria, tuberculosis and meningitis. The intestinal