or eating the wrong kind of food, or working after you are badly
tired, will poison your blood and hinder the proper working of that
beautiful machine, your body. These poisons are made inside your body,
and you can prevent them by living healthfully and wholesomely. But
there are other poisons, which may get into the blood from outside the
body; and while it is best for you not to think too much about these,
or to worry over dangers that may never come, yet it is well to know
just enough about some of them to be able to keep out of their way, as
far as possible.

The most dangerous form of poisons from outside the body are those
made by the germs of some rather common diseases, which, because you
can "catch" them from some one else who has them, are called
"catching," or _infectious_, or _contagious_.

Some of the germs of these "catching" diseases, like the germs of
typhoid fever, of which we have spoken in connection with our drinking
water, are carried in the water or milk that we drink, or upon the
food that we eat; and one of the worst carriers of germs is the
ordinary household fly.

Not so very many years ago, people did not know that _dirt makes
people sick_. You see, they did not know anything about the disease
seeds (germs) that grow so fast in dirt. They did not like to have
flies about, because flies look so dirty and bite people and crawl
over things and spot them. But nowadays, we will not have flies about
because we know that they have been in dirty places where disease
germs live, and that one little fly can carry thousands and thousands
of these germs on his feet.