forces are powerful or
not, according as the general health of the individual is good or bad,
and we see the familiar sight of persons said to be run down taking a
disease, while those not so depleted of vitality are able to resist or
remain immune.

So certain are scientific men of this power and of the fact that the
power resides generally in the white corpuscles of the blood that, in
the presence of a dangerous infection, a person's blood may be examined,
and, if the white corpuscles are not present in sufficient quantity,
proper means must be taken for developing this element in the blood, or
else the person must take himself away from the infection, if the
infection is to be avoided.

As a result of the conflict between the toxins and the defensive forces
of the body, certain vital processes are set free in the blood and in
the cells which seem to possess a highly specialized power of defense
against any subsequent attack. Pasteur, in his researches on the subject
of rabies, developed this power of resistance by inoculating into
rabbits the rabies infection of a monkey. Monkey rabies is not a severe
form and is scarcely felt by the ordinary rabbit, but if the infective
material (usually part of the spinal cord) of the monkey-infected rabbit
is transferred to a second rabbit, the disease becomes more severe; and
if the disease is passed from animal to animal, it may be built up into
as severe a form as desired, up to the maximum. Pasteur found that by
inoculating an individual with a one-day rabbit, that is, with the
weakest brand of infection killing a rabbit in one day, and the next day
with a two-day rabbit, that the person could receive this two-day
inoculation without discomfort or danger because of the greater
antagonism acquired by the preceding inoculation. Continuing the
inoculations for fourteen days and making the strength of the infection
stronger each day, at the end of the period it was found that the
fourteenth inoculation, strong enough to produce the disease and kill a
fresh subject, had, on account of the preceding inoculations, produced
ability to withstand or counteract the actual disease developing perhaps
at the same time. Fortunately, in the case of this disease, the shortest
period for its development is fifteen days, and often it is a month or
more after the bite of the dog before the disease develops. By
successive inoculation of increasing strength for fourteen days, the
system will hav