724 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
found and so successfully applied. But in contrast to this slow, pain- 
ful, 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 cen- 
tury, 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 cura- 
tive 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 augmen- 
tation 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 in- 
gress of the germs in the nonexperimental forms of disease, we con- 
clude from the identity of the conditions present in the experimental 
and nonexperimental 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 bene- 
fit in investigating 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 sometimes those of diphtheria, tuberculosis, and meningitis. The 
intestinal mucous membrane supports a rich and varied bacterial flora 
among which are several potentially harmful species and sometimes, 
even under conditions of health, the bacilli of typhoid fever, of 
