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sick about so many days, then will recover, if the disease is. ;
not so severe as to kill—and this does not often occur—and
afterward the child is immune ; it may be exposed to measles
time and again and will not take it afterward. A person
takes typhoid fever ; if the person is in ordinary health we
can confidently predict that they will be sick about three
weeks ; then a crisis will come, as physicians term it, and
that regular rise and fall of the evening and morning tem-
perature so characteristic of the disease will suddenly break
and the disease has run its course ; if there have not been
tissue injuries that will kill the patient quickly recovers, or if
there have been tissue injuries that hold him down for a
time he will usually recover from these injuries and be as
strong and robust as before. The point is this—that unless
in the course of the particular disease there are tissue in-
juries, or an effect upon the nerve centers during the progress
of the disease that kills, the person recovers and afterward
is immune, generally for life. It is so with yellow fever ; they
are sick for so many days, then if the disease has not been too
severe a crisis comes, the fever disappears and the person re-
covers and afterward is immune. Take the whole group of
these self-limiting diseases and they have this peculiar char-
acter of immunity after one attack. This immunity is differ-
ent in degree. Children who have diphtheria are immune for
a time, but this immunity passes away, the immunity is not
permanent, and in a few others of these microbic diseases the
immunity is not permanent.
Now, there are some notable exceptions to these rules.
Tuberculosis is not a self-limiting disease and immunity does
not follow. Yet I am not certain that that is quite correct, for
those persons who seem to recover from tuberculosis do not
seem to contract it afterward.
Syphilis is not self-limiting and immunity does not oc-
cur, apparently, by reason of having had the disease.
Now, one disease does not immunize against another
yellow fever does not immunize against typhoid fever, or vice
versa ; neither does small-pox immunize against typhoid
fever, and so on. Each one of these immunizes against itself
and itself only. There is a material reason for this immunity.
You will notice that there are several kinds of immunity. We
have natural immunity ; certain animals are naturally immune

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