Because of the wide distribution of contagious disease and the large
number of deaths caused by them, attempts to prevent their spread have
been made from remote times. This has been largely concerned with
isolation or quarantine of sick persons. Doubtless these measures have
been useful, but that they have largely failed to accomplish what is
expected of them is not surprising if we bear in mind that many of
these diseases are most contagious early in their course before they
are recognized and before quarantine is begun, and if we consider
the important part which is played by healthy carriers who are not
suspected and go about freely.
An ideal condition would obtain if it were possible to render all
children, early in life, immune to these diseases. In vaccination
against smallpox we have a measure which has banished smallpox to a
large extent and which, if universally employed, would eradicate the
disease. Similar vaccination measures are now available for diphtheria
and scarlet fever. They are easily carried out, devoid of danger, and
rarely cause even slight discomfort. These will be again referred to in
discussing the individual diseases.