THE NATURE OF INFECTION.--THE INVASION OF THE BODY FROM ITS
SURFACES.--THE PROTECTION OF THESE SURFACES.--CAN BACTERIA PASS
THROUGH AN UNINJURED SURFACE.--INFECTION FROM WOUNDS.--THE WOUNDS IN
MODERN WARFARE LESS PRONE TO INFECTION.--THE RELATION OF TETANUS TO
WOUNDS CAUSED BY THE TOY PISTOL.--THE PRIMARY FOCUS OR ATRIUM OF
INFECTION.--THE DISSEMINATION OF BACTERIA IN THE BODY.--THE DIFFERENT
DEGREES OF RESISTANCE TO BACTERIA SHOWN BY THE VARIOUS ORGANS.--MODE
OF ACTION OF BACTERIA.--TOXIN PRODUCTION.--THE RESISTANCE OF THE BODY
TO BACTERIA.--CONFLICT BETWEEN PARASITE AND HOST.--ON BOTH SIDES MEANS
OF OFFENSE AND DEFENSE.--PHAGOCYTOSIS.--THE DESTRUCTION OF BACTERIA BY
THE BLOOD.--THE TOXIC BACTERIAL DISEASES.--TOXIN AND
ANTITOXIN.--IMMUNITY.--THE THEORY OF EHRLICH.


As has been said, infection consists in the injury of the body by
living organisms which enter it. The body is in relation to the
external world by its surfaces only, and organisms must enter it by
some one of these surfaces. It is true that the bacteria in the
intestine--either those normally present or unusual varieties--may,
under certain circumstances, produce substances which are injurious
when absorbed; but this is not infection, and is analogous to any
other sort of poisoning. Each surface of the body has its own
bacterial flora. Organisms live on the surface either on matter which
is secreted by the surface or they use up an inappreciable amount of
body material. Many of these bacteria are harmless, some are
protective, producing by their growth such changes in the surface
fluids that these become hostile to the existence of other and
pathogenic forms. The surfaces also frequently harbor pathogenic
organisms which await some condition to arise which will permit them