no
special importance, but if it is introduced into the body through a
scratch or any other wound it becomes a very serious matter.

We may say, then, that the effect the bacillus has on the host depends
largely on the host. Not only does it depend on what the host is, but
the particular condition of the host at the time of infection is of
importance. Children are subject to many diseases that adults seldom
have. Hunger, thirst, fatigue, exposure and other factors may make a
person susceptible to the actions of certain bacteria that would be
harmless under other conditions.

The minute size and great numbers of the bacteria make their
dissemination a comparatively simple matter. They may be carried in the
air as minute particles of dust; they may be carried in water or milk;
they may be carried on the clothing or on the person from one host to
another, or they may be disseminated in scores of other ways. In other
chapters, particularly the one dealing with the house-fly and typhoid,
we shall see how it is that insects are often important factors in
spreading some of the most dreaded of the bacterial diseases.

THE PROTOZOA

The Protozoa, or one-celled animals, belonged to an unknown world before
the invention of the microscope. The first of these instruments enabled
the early observers to see some of the larger and more conspicuous
members of the group and each improvement of the microscope has enabled
us to see more and more of them and to study in detail not only the
structure but to follow the life-history of many of them.

_The Amoeba._ With some, as the common amoeba (Fig. 8), a minute
little form that is to be found in the slime at the bottom of almost any
body of water, the life-history is extremely simple. The organism itself
consists of a minute particle of protoplasm, a single cell with no
definite shape or body-wall and no specialized organs or apparatus for
carrying on the life-functions. It lives in the slime or ooze in fresh
or salt water, takes its food by simply flowing over the particle that
is to be ingested, grows to a certain limit of size, then divides into
two more or less equal parts, each part becoming a new animal that goes
on with its development as did the parent form. This process of growth
and division may go on for many generations, but cannot continue
indefinitely unless there is a conjugation of two separate individuals.
This process of conjugation is just the opposite to that of