ing about decomposition of various kinds. Thus a
highly organized fungus, like the yeast plant, growing in the presence of
sugar, has the power of breaking down this complex body into simpler ones,
_viz._, alcohol and carbon dioxid.

In the same way, various forms of bacteria have the power of breaking down
complex bodies in their immediate neighborhood, the products depending
upon the substance, the kind of bacteria, and the conditions under which
they act. Thus the _bacteria lactis_ act upon the milk sugar present in
milk, and convert it into lactic acid, thus bringing about the souring of
milk.

[Illustration: Fig. 169.--Examples of Pathogenic Bacteria. (Drawn from
photographs.)

A, spiral form of bacteria found in cholera (Magnified about 1000
diameters)
B, rod-shaped bacteria (called _bacilli_) from a culture obtained
in _anthrax_ or malignant fustule of the face. Diseased hides
carry this micro-organism, and thus may occasion disease among those
who handle hides and wool. (Magnified about 1000 diameters)
]

Now, while most species of bacteria are harmless, some are the cause of
sickness and death when they gain admittance to the body under certain
conditions. These disease-producing bacteria (known as _pathogenic_), when
established in the blood and tissues of the body, bring about important
chemical changes, depending upon the species of bacteria, and also produce
a particular form of disease. The production of certain diseases by the
agency of bacteria has now been proved beyond all doubt. In yellow fever,
erysipelas, diphtheria, typhoid fever, consumption and other diseases, the
connection has been definitely established.

The evil results these germs of disease produce vary greatly in kind and
severity. Thus the bacteria of Asiatic cholera and diphtheria may destroy
life in a few hours, while those of consumption may take years to produce
a fatal result. Again, the bacteria may attack some particular organ, or
group of organs, and produce mostly local symptoms. Thus in a boil there
is painful swelling due to the local effect of the bacteria, with slight
general disturbance.

398. The Battle against Bacteria. When we reflect upon the terrible
ravages made by infectious diseases, and all their attendant evils for
these many years, we can the better appreciate the work done of late years
by tireless scientists in their efforts to modify the activity of
disease-producing bacteria. I