Some of these infectious diseases are so common among children that
they are called _Children's Diseases_, or the _Diseases of Infancy_,
just as if it were natural for you to have them while you are
children, and as if they were something that you have to have as a
matter of course, before you grow up.

But it isn't necessary at all to have them, if you will take care of
yourselves and help your doctors and the Board of Health of your
county or town or city to prevent their spreading. These diseases,
although usually very mild, never do anyone any good whatever, and may
do serious harm; for their poisons may stay in the blood and injure
the heart or the kidneys or the nerves.

One thing I should like to urge you to do if you happen to get one of
these "children's diseases"; and that is, to stay in bed or out of
school or away from work just as long as your doctor tells you to.
This is important, because it is very dangerous indeed to become
over-tired or overheated or chilled, or to get your feet wet or romp
too hard or sit up too late, before you have fully recovered; and you
will not have fully recovered until at least three or four weeks after
you are able to be out of bed. But if you take good care of yourselves
for three or four weeks after measles or chicken pox or whooping cough
or a very bad cold, you will avoid almost all danger of their poisons
injuring your heart or kidneys or nerves, and causing chronic
diseases, like Bright's disease or heart disease, later in life.

Perhaps now I have told you enough about poisons and sickness. You
must not be frightened about them. I have told you these things so
that you may understand why you must bathe, and brush your teeth, and