lurking in the soil. Leprosy
is communicated from a leper in the same way. The almost ubiquitous
bacteria of blood-poisoning (septicaemia) may enter by the smallest
fissure of the skin, still more readily by large cuts or wounds. The
bites and stabs of small and large animals--wolves, dogs, flies,
gnats, fleas and bugs, also open the way, and often the deadly microbe
has associated itself with the biting animal and is carried by it,
ready to effect an entrance. Thus rabies (hydrophobia) is introduced
by the bites of wolves and dogs, and a whole series of diseases, such
as plague, malaria, sleeping-sickness, gaol-fever (typhus), yellow
fever, relapsing fever, and others, are introduced into the human body
by blood-sucking insects. Hence the immense importance of treating
every slightest wound and scratch with chemicals (called
"antiseptics"), which at once destroy the invading microbe--and of
keeping a wounded surface covered and protected from their approach.
In ways at one time unsuspected, such openings may be made by which
poisonous microbes enter the body. Thus the little hard-skinned
parasitic thread-worms which are often brought in by uncooked food
into man's intestine, though by themselves comparatively harmless,
scratch the soft lining of the bowel and enable poison-making microbes
to enter the deeper tissues, and cause dangerous abscesses and
appendicitis.

The carriers of disease germs thus become a very important subject of
study. There are carriers which make no selection, but are, so to
speak, "casual" in their proceedings, and there are others which have
the most special and elaborate relations to some one kind of
disease-causing microbe for which alone they are responsible, and to
the life of which they are necessary. Let us look first at the more
casual group. Man himself is a great carrier and distributor of his
own diseases. Unless and until he has learned to be careful and guard
against thoughtless proceedings, he is always spreading the microbes
of his diseases and passing them on to his fellow men. He pollutes the
waters, rivers, lakes, and pools from which others drink. He manures
his crops, and then eats some of them uncooked. His hands are polluted
by disease-causing microbes, and he handles (to an alarming and
unnecessary extent) the food, such as bread and fruit, which is
swallowed by his fellows, without cleansing it by heat. It has lately
been shown that apparently healthy men and women oft