equate. For instance, the bacteria of smallpox
has never been found, although the disease is so characteristically one
of bacterial origin that no one can doubt the cause. Similarly, the
bacteria responsible for measles, scarletina, and whooping cough have
never been discovered, although the cause of each is also presumably
bacterial. More definite information on the subject of the individual
and responsible bacteria will be given in the subsequent chapters
dealing with specific diseases. Inquiries into the method of growth and
into the life history of specific bacteria serve our present purpose
only as they teach methods for the prevention of the disease. For
example; when it was found that the parasite of yellow fever, in the
course of its life, spent fourteen days in the mosquito's body in such a
condition that the mosquito during that time was harmless, it made
possible exposure to mosquitoes laden with yellow fever for a period of
thirteen days from the time of the preceding case.

_Antitoxins._

But the methods of combating the different diseases when once contracted
in the human body, based on the knowledge obtained of the life history
of these germs, have been the most important result of their biological
study. A large part of this knowledge has been acquired by the study of
animals which have been found susceptible and so available for
experimental investigation, and it may be that the impossibility of
studying measles, for instance, in animals, may be one reason why the
germ has never been discovered.

There is no evidence that animals suffer spontaneously from such
diseases as typhoid fever, Asiatic cholera, leprosy, yellow fever,
smallpox, measles, and so on; but it seems that in animals, as in man,
the disease is the direct result of the life and growth in the animal of
the characteristic disease-producing germ. The fact that diphtheria or
tuberculosis can be experimentally given to rabbits or guinea pigs is
without doubt the chief source of our knowledge of those diseases,
although, in general, it is impossible to produce diseases in any animal
which will be, clinically, precisely like the disease as it appears in
man. The converse of this is also true, namely, that when it has been
found impossible to experimentally inoculate an animal with a disease
supposed to be bacterial in nature, then but very little of that disease
is known.

The most important result of bacterial studies has been the producti