IMMUNITY AND SUSCEPTIBILITY. • 67

field-mouse, and white mouse differ very much in their
susceptibility to various diseases.

Acquired immunity is resistance which is the result
of accidental circumstances. It may result—

A. By recovery from a mild attack of the disease.
Most adults have suffered from rubeola, scarlatina, and
varicella in childhood, and in consequence of the attacks
are now immune to these diseases—/. e. will not become
affected again. One attack of yellow fever is always a
complete guard against another. Typhoid fever is rarely
followed by a second attack.

B. By recovery from an attack of a slightly different
disease. Sometimes the immunity is experimentally pro-
duced, as when by vaccination we produce the vaccine
disease and afterward resist variola. Acquired immunity
is a little less complete and not so permanent as natural
immunity, for in the latter it is only when the functions
of the individual are disturbed or his vitality depressed
that the resistance is lost, while in the former time seems
to lessen the power of resistance, so that rubeola and
scarlatina may return in a few months or years, and for
complete protection vaccination may need to be done as
often as every seven years.

C. By the injection of antitoxic substances. At
present there is much agitation over the newly-dis-
covered antitoxin of diphtheria, the injection of about
500 units of which will give complete protection against
the disease for a period lasting from a month to six
weeks.

Immunity may be destroyed in numerous ways:
(a) By variation from tJie normal temperature of the
animal under observation. Pasteur observed that chick-
ens would not take anthrax, and suspected that this
immunity might be due to their high body-temperature.
After inoculation he plunged the birds into a cold bath,
reduced their temperature, and succeeded in destroying
their immunity. The experiment was a success, but the
reasoning seems to have been fault}', as the sparrow,