e careful about putting things
into the mouth, and about the cleanliness of the skin. We often take a
few disease germs into the body without catching a disease. This is
because the white cells of our blood fight the germs and kill them (p.
53). If the body is hurt or weakened, the white blood cells may also
be weakened so that they cannot kill the germs. We should take good
care of the body so that every part of it may do its work well. We
need not be able to run fast, or to lift heavy weights, but the best
sign that every part of the body is in good order is to feel bright
and wide-awake. Then our white blood cells will also be in good order
and able to fight disease germs.

=213. Catching cold.=--When we catch a disease, we often say that we
have caught cold. We used to think that cold air and dampness were
almost the only causes of taking cold, and this is the reason why we
called many kinds of sickness by the name of colds. Now we know that
we catch cold by taking disease germs into the body. The germs will
not be able to grow unless the body is weakened in some way, as by
cold and dampness. Yet if we are wet and cold, we shall not catch cold
unless we take disease germs into the body. We do not get the germs
from the outdoor air, for very few germs are there. We get them from
the foul air of our houses when we come in to warm and dry ourselves.
If the air of our houses were always as clean and pure as the outdoor
air, we should hardly ever have colds.

We can safely let the cold air blow on us if we are out of doors, but
if we sit in a house, a small draft sometimes seems to make us take
cold. This is because there are likely to be many disease germs in the
house and few out of doors.

Other things besides cold air and dampness may weaken the body, and so
help us to take cold. If germs of colds are in a warm room, we may sit
there and take cold even if we are not wet or chilled at all. The body
may be weakened by poor food, wrong eating, or overwork, so that
disease germs will easily grow in it. We take as many colds from these
causes as from cold air and dampness.

=214. Kinds of colds.=--A person takes most of the germs of colds
through his nose and mouth. If they grow only in his nose, we say that
he has a cold in his head. If they grow in his throat, he has a sore
throat, or tonsillitis. If they reach as far as the upper part of his
windpipe, he is hoarse, or has a cough, or the croup. If the germs are
pl