6 THE POPULAR SCIENCE MONTHLY 



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 suc- 

 cessfully applied. But in contrast to this slow, painful and halting 

 advance in practical means for the relief of suffering, is to be placed 

 the bod} r 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 accomplish- 

 ment. 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 inva- 

 sion, 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 pre- 

 determined 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 investi- 

 gating disease. 



To escape disease is not merely to remain without the zone of in- 

 fluence 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 some- 

 times those of diphtheria, tuberculosis and meningitis. The intestinal 



