thought to be infectious are now classed as such
because they have been proved to be caused by living germs. Conspicuous
among these diseases is pulmonary consumption. In the case of almost
every one of these diseases we have discovered the specific germ and are
able to demonstrate its presence, either by its microscopical
appearance, by its behavior on contact with certain stains, or by the
forms that cultures of it assume. The micro-organism of small-pox and
that of cancer (the existence of which is assumed) have not yet been
isolated. Some of these germs, like that of tetanus (lockjaw), gain
entrance to the system only through a wound; others, like those of
typhoid fever and cholera, are swallowed; others, like that of
pneumonia, are inhaled; still others, like that of tuberculous disease,
are either swallowed or inhaled. Some are believed to be transmissible
to the unborn child; and a few are ordinarily harmless parasites,
becoming pathogenic only when they accidentally gain access to other
parts of the system than those which constitute their natural habitat.

These microscopic organisms do not by their mere presence set up
disease, unless indeed they are in such overwhelming numbers as to block
the capillary blood vessels mechanically. Some of them are carried
broadcast in the blood current, while others remain at the point of
entrance; in either case they elaborate certain products, termed
toxines, which act, either locally or through the circulation, to cause
the disease. These toxines eventually kill the micro-organisms that
produced them, quite as an animal may be smothered in its own
exhalations; or at least they would do so if the "host" survived long
enough for the completion of the process. Meantime, they have either
killed the "host" or been defeated by certain very interesting natural
processes. But before either of these occurrences has had time to take
place, fortunately, in the great majority of instances, save those of
exposure to the most deadly of infections, the vital power of the
invaded individual has coped successfully with the invaders at the very
point of attack--has repulsed the attacking party without appreciable
impairment of its own force--and no illness results. For example,
practically all of us inhale the germ of consumption repeatedly, but
most of us suffer no harm from it simply because the fluids which bathe
the surface on which the germ effects a lodgment are endowed with
properti