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 important 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 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 remarkable anti-bacterial power of the blood was demonstrated directly by injecting