The number of bacteria originally present makes a difference also. It
is readily understood that the tissues and white blood cells would
find it more difficult to repel the invasion of an army of a million
microbes than the attack of a squad of ten similar fungi. I have said
that the experimenter can weaken and augment the virulence of bacteria
by manipulating their surroundings in the laboratory. It is probable
that such a change occurs in nature. If so, some bacteria are more
virulent than others of the same species; some less virulent. A few of
the less virulent disposition would be more readily killed by the
white cells and tissues than would a larger number of the more
virulent ones. At other times the danger from microbic infection is
greater because there are two species introduced at the same time; and
these two multiply more vigorously when together than when separated.
There are, in fact, two allied hosts trying to destroy the blood cells
and tissues. This occurs when the bacteria of putrefaction and the
bacteria of suppuration are introduced into the tissues at the same
time. The former cause sapræmia and septicæmia, the latter cause
suppuration. The bacteria of tuberculosis are said to act more
viciously if accompanied by the bacteria of putrefaction.
Osteomyelitis is of greater severity, it is believed, if due to a
mixed infection with both the white and golden grape-coccus of
suppuration.

I have previously mentioned that the bacteria of malignant pustule are
powerless to do harm when the germs of erysipelas are present in the
tissues and blood. This is an example of the way in which one species
of bacteria may actually aid the white cells, or leucocytes, and the
tissues in repelling an invasion of disease-producing microbes.

Having occupied a portion of the time allotted to me in giving a crude