some of the antitoxines may be
efficacious against more than one kind of toxine, for there are
physicians who are convinced that vaccination is a temporary preventive
of whooping-cough. But the elaboration of an antitoxine takes time, and
the result in any given case, whether in recovery or in death, seems to
be settled by the ability or inability of the vital powers of the
individual to hold out until they are relieved by the evolution of the
necessary amount of antitoxine.

In the long run, provided the sick person survives, more antitoxine is
generated than is required to save life. The excess remains in the
system for a greater or lesser length of time, and this fact explains
the individual's subsequent immunity to the disease from which he has
recovered; any fresh invading force of the microbes of that disease
finds that defensive preparations have been made in advance. In the case
of some diseases this acquired immunity is usually lifelong, as in that
of small-pox; in others, of which influenza is a notable example, it is
as a rule very transitory; and there are all gradations between the two.
It is thought that this acquired immunity to some diseases may be
transmitted to the offspring, for it is quite certain that there are
many people who are from birth insusceptible to scarlet fever, no matter
what may be the extent of their exposure to that disease.

The recognition of Nature's elaboration of protective antitoxines has
led to their artificial cultivation in the lower animals, and, thus
produced, they have been used with brilliant results in the prevention
and cure of at least one formidable disease, diphtheria. The immense
reduction of the mortality from this disease that has followed the
introduction of the treatment with the artificial antitoxine we owe to
Behring, of Germany, and Roux, of France. Omitting unnecessary details,
we may describe the process of obtaining diphtheria antitoxine as
follows: A certain amount of diphtheritic poison (of the bacteriological
sort, prepared by cultivating the diphtheria microbe) is injected into
the circulation of a horse--sufficient to make the horse sick, but not
enough to endanger his life. The horse's system straightway begins to
elaborate the protective antitoxine, and there results from this one
injection a sufficient amount of it to save the horse, although far too
little to make the serum of his blood potent enough for medicinal use.
Hence, after the lapse of