Diphtheria is very contagious, and many people, especially children, can
catch it. For this reason, whenever a case of diphtheria is discovered,
the doctor injects the antitoxin not only into the patient, but also, as
a protective against the disease, into those who have come into contact
with the patient. This is spoken of as "immunizing" these individuals.
The immunizing dose is not so large as the curative dose given to the
patient, but it is usually sufficient to protect those exposed to
diphtheria for a month from the time of injection. At the end of that
time the protection disappears.