sults of processes in the system that
are nearly analagous to fermentation, and that such diseases are
therefore traceable ultimately to the existence of living germs.

This view of the case brought Pasteur to a large and general
investigation of bacteria. The bacterium may be defined as a
microscopic vegetable organism; or it may be called an _animal_
organism; for in the deep-down life of germs there is not much
difference between vegetable and animal--perhaps no difference at all.
The bacterium is generally a jointed rod-like filament of living
matter, and its native world seems to be any putrefying organic
substance.

Bacteria are the smallest of microscopic organisms. They are widely
diffused in the natural world, existing independently and also in a
parasitical way, in connection with larger forms of organic life. They
multiply with the greatest rapidity. On the whole, the bacterium
fulfills its vital offices in two ways, or with two results; first,
_fermentation_, and secondly, _disease_.

To this field of inquiry Pasteur devoted himself with the greatest
assiduity. He began to investigate the diseased tissue of animals, and
was rewarded with the discovery of the germs from which the disease
had come. It was found that the bacteria of one disease are different
from those of another disease, or in a word that the microscopic
organisms which produce morbid conditions in animals are
differentiated into genera and species and varieties, in the same
manner as are the animals, birds and fishes, of the world. A new realm
of life invisible save by the aid of the microscope, began to be
explored, and practical results began to follow.

Pasteur at length announced his ability to _produce_ infectious
diseases by inoculation; and of this his proofs and demonstrations,
were complete. In the next place he announced his ability to
_counteract_ the ravages, of certain classes of diseases (those called
zymotic) by inoculating the animal suffering therefrom with what he
called an "attenuated" or "domesticated" virus of the given disease.

The matter first came to a practical issue by the inoculation of well
animals with the attenuated virus. The animals so treated became
_immune_; that is, exempt from the infection of the given disease.
Pasteur gave public demonstrations in the fields near Paris, using the
disease called splenic fever, and sheep as the subjects of his
experimentation. The whole civilized world was astonished