vital activities.

If the surroundings are too hot, too cold, or too dry, or if they are
not supplied with a proper quantity and quality of food, the bacterium
becomes inactive until the surrounding circumstances change; or it may
die absolutely. The spores, which finally become full-fledged
bacteria, are able to stand a more unfavorable environment than the
adult bacteria. Many spores and adults, however, perish. Each kind of
bacterium requires its own special environment to permit it to grow
and flourish. The frequency with which an unfavorable combination of
circumstances occurs limits greatly the disease-producing power of the
pathogenic bacteria.

Many bacteria, moreover, are harmless and do not produce disease, even
when present in the blood and tissues. Besides this, the white blood
cells are perpetually waging war against the bacteria in our bodies.
They take the bacteria into their interiors and render them harmless
by eating them up, so to speak. They crowd together and form a wall of
white blood cells around the place where the bacteria enter the
tissue, thus forming a barrier to cut off the blood supply to the
germs and, perhaps, to prevent them from entering the general blood
current.

The war between the white blood cells and the bacteria is a bitter
one. Many bacteria are killed; but, on the other hand, the life of
many blood cells is sacrificed by the bacteria poisoning them with
ptomaines. The tissue cells, if healthy, offer great resistance to the
attacks of the army of bacteria. Hence, if the white cells are
vigorous and abundant at the site of the battle, defeat may come to
the bacteria; and the patient suffer nothing from the attempt of these