hanged
its character, but that such diseases as smallpox, syphilis, and
certain forms of tuberculosis were formerly confounded with leprosy,
thus giving a false idea of its prevalence.

In certain cases the adaptation of the organism is for a narrow
environment; for example, the parasitism may extend to a simple
species only, in others the adaptation may extend to a number of
genera. In certain cases the adaptation is mutual, extending to both
parasite and host and resulting in symbiosis, and this condition may
be advantageous for both. Certain of the protozoa harbor within them
cells of algae utilizing to their own advantage the green chlorophil of
the algae in obtaining energy from sunlight and in turn giving
sustenance to the algae. Although the algae are useful guests, when they
become too numerous the protozoan devours them. It is evident that
symbiosis is the most favorable condition for the existence of the
parasite, and an injurious action exerted by the parasite on the host
unfavorable. The death of the host is an unfortunate incident from the
parasite's point of view in that it is deprived of habitation and food
supply, being placed in the same unfortunate situation as may befall a
social parasite by the death of his host.

FOOTNOTE:
[1] Flexner has recently succeeded in isolating and cultivating the
organism of poliomyelitis, but the organism is so small that its
classification is not possible.

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. I