t may
be stated here that, as a general rule, the virulence of an organism
towards an animal is increased by sojourn in the tissues of that animal.
The increase of virulence becomes especially marked when the organism is
inoculated from animal to animal in series, the method of _passage_. This
is chiefly to be regarded as an adaptation to surroundings, though the fact
that the less virulent members of the bacterial species will be liable to
be killed off also plays a part. Conversely, the virulence tends to
diminish on cultivation on artificial media outside the body, especially in
circumstances little favourable to growth.

[Sidenote: Immunity.]

By immunity is meant non-susceptibility to a given disease, or to
experimental inoculation with a given bacterium or toxin. The term must be
used in a relative sense, and account must always be taken of the
conditions present. An animal may be readily susceptible to a disease on
experimental inoculation, and yet rarely or never suffer from it naturally,
because the necessary conditions of infection are not supplied in nature.
That an animal possesses natural immunity can only be shown on exposing it
to such conditions, this being usually most satisfactorily done in direct
experiment. Further, there are various degrees of immunity, and in this
connexion conditions of local or general diminished vitality play an
important part in increasing the susceptibility. Animals naturally
susceptible may acquire immunity, on the one hand by successfully passing
through an attack of the disease, or, on the other hand, by various methods
of inoculation. Two chief varieties of artificial immunity are now
generally recognized, differing chiefly according to the mode of
production. In the first--_active immunity_--a reaction or series of
reactions is produced in the body of the animal, usually by injections of
bacteria or their products. The second--_passive immunity_--is produced by
the transference of a quantity of the serum of an animal actively immunized
to a fresh animal; the term is applied because there is brought into play
no active change in the tissues of the second animal. The methods of active
immunity have been practically applied in _preventive inoculation_ against
disease; those of passive immunity have given us _serum therapeutics_. The
chief facts with regard to each may now be stated.

1. _Active Immunity_.--The key to the artificial establishment of active
immunity is given