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of these diseases have any self-originating power; they
are not generated in the system by any abnormal
processes of nutrition or secretion; they are not begotten
of dirt, poverty, or neglect, though these greatly aid
their extension; they are the product of a distinct virus,
conveyed directly or indirectly from one person to another,
and they manifest their potency and individuality
by producing the same diseases in others. Of them it
may be said, to parody the aphorism of the homoeopath,
similia similibus creantur, or to be more precise,
eadem iisdem creantur. As Sir James Simpson has
remarked, "We would no more expect this known
species of disease or poison to originate de novo at the
present day, under any combination of circumstances,
than we would expect a known species of animal or
plant, as a dog or a hawthorn to spring up de novo,
and without antecedent parentage."
Time was, when the occurrence of these diseases—
children's diseases as they are termed—was expected
with the certainty, if not with the regularity of teething.
They were regarded as necessarily incidental to childhood.
It may be that measles and scarlet fever were
then of a milder type, and that an attack from either of
them was accepted as a matter of course, and caused
very little anxiety. The latter disease, however, seems
to have acquired with time such an intensity of virulence—with
an occasional capriciousness in its malignity—that
it nowoften desolates whole families, and has
come to be regarded as the plague of modern days. Of
the ravages of small-pox, those who have passed life's
meridian do not require to be informed. But Jenner