by the fact long established that recovery from an attack
of certain infective diseases is accompanied by protection for varying
periods of time against a subsequent attack. Hence follows the idea of
producing a modified attack of the disease as a means of prevention--a
principle which had been previously applied in inoculation against
smallpox. Immunity, however, probably results from certain substances
introduced into the system during the disease rather than from the disease
itself; for by properly adjusted doses of the poison (in the widest sense),
immunity may result without any symptoms of the disease occurring. Of the
chief methods used in producing active immunity the first is by inoculation
with bacteria whose virulence has been diminished, _i.e._ with an
"attenuated virus." Many of the earlier methods of attenuation were devised
in the case of the anthrax bacillus, an organism which is, however,
somewhat exceptional as regards the relative stability of its virulence.
Many such methods consist, to speak generally, in growing the organism
outside the body under somewhat unsuitable conditions, _e.g._ at higher
temperatures than the optimum, in the presence of weak antiseptics, &c. The
virulence of many organisms, however, becomes diminished when they are
grown on the ordinary artificial media, and the diminution is sometimes
accelerated by passing a current [v.03 p.0176] of air over the surface of
the growth. Sometimes also the virulence of a bacterium for a particular
kind of animal becomes lessened on passing it through the body of one of
another species. Cultures of varying degree of virulence may be obtained by
such methods, and immunity can be gradually increased by inoculation with
vaccines of increasing virulence. The immunity may be made to reach a very
high degree by ultimately using cultures of intensified virulence, this
"supervirulent" character being usually attained by the method of _passage_
already explained. A second method is by injection of the bacterium in the
dead condition, whereby immunity against the living organism may be
produced. Here manifestly the dose may be easily controlled, and may be
gradually increased in successive inoculations. This method has a wide
application. A third method is by injections of the separated toxins of a
bacterium, the resulting immunity being not only against the toxin, but, so
far as present knowledge shows, also against the living organism. In the
developme