RESISTANCE TO INFECTIOUS DISEASE 7 



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 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 powerless 

 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 exclusion of the 

 bacteria from the interior of the body, although in itself this is an im- 

 portant means of protection for which special mechanisms 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 small-pox, 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 small-pox, which in 

 ante- vaccination days overlooked few if any persons in infected regions, 

 a great diversity of susceptibility to infection has been noted again and 

 again among exposed persons and animals. This variability of infec- 

 tivity 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 

 othe^; 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 unat- 

 tained ideal. 



It was early apparent that bacteria must sometimes escape into the 

 blood and yet that infection did not follow. It was observed that fre- 

 quently 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 remarkable anti- 

 bacterial power of the blood was demonstrated directly by injecting 



