NATURAL RESISTANCE TO INFECTIOUS DISEASE AND 
ITS REINFORCEMENT.2 
By Simon FLexner, M. D., 
Rockefeller Institute for Medical Research. 
Common observations early indicated that individuals of all animal 
species, and of the human species especially, were very unequally sub- 
ject to disease. This elementary fact is impressed every, day upon 
the thoughtful and has been, from the earliest times, the object of 
much ingenious speculation. Even to-day, and in spite of the ac- 
quisition. of a wealth of new facts in physiology and pathology, we 
are not able to define fully the conditions that make for or against 
disease. However, the new knowledge which has been acquired en- 
ables us to see much more deeply and clearly into the complex mech- 
anisms of disease than could be seen half a century ago; but unfor- 
tunately our insight has not been strengthened as regards all diseases, 
but almost exclusively in relation to the infectious diseases. In re- 
spect to the other class, or noninfectious or chronic diseases, among 
which are Bright’s disease, vascular disease, malignant tumors, the 
gains in fundamental knowledge are far less great. 
It may be axiomatic to state that all actual progress in unraveling 
the complicated conditions of disease depends upon precise knowledge 
of its underlying causes; and yet in an age in which comparative 
ignorance still requires that a certain amount of practice shall be 
empirical, it is well to bear in mind this notion, so that what is under- 
taken through knowledge ‘may be kept distinct from what is adven- 
tured through ignorance. It has been to the lasting credit. of the 
medical profession of an early period, when actual knowledge of the 
underlying causes of disease had not, ade in the then state of develop- 
ment of the physical sciences could not, have yielded a single concrete 
fact, that one ehadee nation ia the most perfect one yet dis- 
covered of preventing a disease, and two drugs—quinine and mer- 
cury—specific for two other infectious diseases, should havé been 
“Read at the university lectures on public health at Columbia University, 
New York City, March 1. Reprinted, by permission, from The Popular Science 
Monthly, July, 1909. 
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