NATURAL RESISTANCE TO DISEASE—-FLEXNER. (As 
dysentery, and in regions in which cholera is endemic, or during its 
epidemics, of cholera bacilli. 
It is obvious, therefore, that it is practically impossible to escape 
the dangers of bacterial infection, and withdrawal absolutely from 
other human beings and from all human habitations would be power- 
less to accomplish this result. It is equally obvious that with such 
constant and universal exposure to bacterial infection the body must, 
for the greater part, easily defend itself against this class of its 
enemies. It is now known that this defense is not merely by exclu- 
sion of the bacteria from the interior of the body, although in itself 
this is an important means of protection for which special mechan- 
isms are provided, but that constant small escapes of bacteria into the 
blood are taking place from the mucous membranes chiefly, and that 
there rarely ensues disease from this cause. 
On the other hand, there is another class of disease germs that do 
not regularly inhabit the body and whose influence is occasional only. 
Some of these germs are exquisitely infectious, as, for example, 
those causing smallpox, measles, and scarlet fever; and others require 
an intermediate agency to inoculate them as in malaria, yellow fever, , 
and possibly bubonic plague. And yet, excluding smallpox, which in 
ante-vaccination days overlooked few if any persons in infected re- 
gions, a great diversity of susceptibility to infection has been noted 
again and again among exposed persons and animals. This variabil- 
ity of infectivity affects difference in species, race, and individuals 
and constitutes one of the fundamental problems of disease. Certain 
diseases are naturally limited to certain species and can not at all, or 
can only with great difficulty, be transferred to another, although 
related species; other diseases appear among: several species widely 
separated from each other; still other diseases choose by preference 
or are quite restricted to certain breeds of a species; and finally, 
individuals of a homogeneous species exhibit wide differences of 
susceptibility to infection. A worked-out theory of infection to and 
immunity from disease would include and explain all these and 
many more diversities which have been observed. I need not offer 
an apology for this at present unattained ideal. : 
It was early apparent that bacteria must sometimes escape into the 
blood and yet that infection did not follow. It was observed that 
frequently at death the interior of the body was free of bacteria and 
might remain so for many hours and until signs of putrefaction began 
to be apparent. The deduction from this observation was to the 
effect that the blood and organs must’ protect themselves during life 
and for a period after death from bacterial development. The re- 
markable antibacterial power of the blood was demonstrated directly 
by injecting putrescent fluids into the veins of rabbits and noting that 
not only might they survive the infections and remain quite normal, 
