CHAPTER VII.
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.