e.
Chance enters into this, but plays a small part, for the same varying
individual susceptibility is shown experimentally. If a given number
of animals of the same species, age and weight, even those from the
same litter, be inoculated with a given number of bacteria shown to be
pathogenic for that species, the results differ. If the dose be
necessarily fatal, death will take place at intervals; if a dose
smaller than the fatal be used, some animals will die, others will
recover. The defences of the organism being centred in the activity of
the living tissue, any condition which depresses cell activity may
have an effect in increasing susceptibility to infection. Animals
which ordinarily are not susceptible to infection with a certain
organism may be made so by prolonged hunger, or fatigue, by the
influence of narcotics, by reduction of the body temperature, by loss
of blood. In man prolonged fatigue, cold, the use of alcohol to excess
and even psychic depression increases susceptibility. It has been
shown that such conditions are accompanied by a diminution in the
power of the blood to destroy bacteria.

There is variation in the susceptibility to infection in the different
races of man. If a race be confined to one habitat with close
intercourse between the people, such a race may acquire a high degree
of immunity to local diseases by a gradual weeding out of the
individuals who are most susceptible. A degree of comparative harmony
may be gradually established between host and parasite, as is the case
in wild animals. These have few diseases, the weak die, the resistant
breed; they harbor, it is true, large numbers of parasites, but there
is mutual adjustment between parasite and host. Diseases in animals
greatly increase under the artificial conditions of domestication.
Certain highly specialized breeds of cattle, as the Alderneys, are
much more susceptible to tuberculosis than the less specialized. The
high development of the variation which consists in a marked ability
to produce milk fat is probably combined with other qualities, shown
in diminished resistance to disease, and under natural conditions the
variation would not have persisted. The introduction of a new disease
into an isolated people has often been attended with dire
consequences. It is much the same thing with the introduction of
disease of plants. In Europe the brown-tail moth and the gypsy moth
produce continuously a certain amount of damage to t