Each of these diseases is caused by its own peculiar germ. We may
compare the germs causing contagious diseases to seeds of plants.
Each variety of seed will produce only the sort of plant from which
it came. Each of these diseases is due to the implantation of its own
peculiar germ in the body, and except in this way the disease never
occurs. Each case thus originates from a previous case of the same
sort. The old idea that contagious diseases are caused by sewer gas,
bad air, disturbances in the weather or similar things, is now known
to be untrue. The germs causing diphtheria, scarlet fever and whooping
cough and perhaps measles have been isolated and studied. They are all
bacteria, which are very small vegetable organisms. In order to be seen
by the human eye, they must be magnified about one thousand times by a
microscope.
As seeds must be placed in suitable soil if they are to grow and
produce plants, so disease germs must find a suitable soil in the body
in order to cause disease.