the process by
which almost all known diseases, except a few untamable hyenas, like the
Black Death, the cholera, and smallpox, have gradually grown milder with
civilization. If we escape the attack of these attenuated diseases of
infancy until fifteen or sixteen years of age, we can usually defy them
afterward; though occasionally an unusually virulent strain will attack
an adult, with troublesome consequences.

At all events, whatever explanation we may give, the consoling fact
stands out clearly that civilized man is decidedly more resistant to
these pests of civilization than is any half-civilized race, and there
is good reason to believe that this is a typical instance of his
comparative vigor and endurance all along the line.

If this view of the original character and taming of these diseases be
correct, it also accounts for the extraordinary and otherwise
inexplicable cases where they suddenly assume the virulence of cholera,
or yellow fever, and kill within forty-eight or ninety-six hours, not
merely in children but also in adults.

To group these three diseases together simply because they all happen to
occur in children would appear scarcely a rational principle of
classification. Yet, practically, widely different as they are in their
ultimate results and, probably, in their origin, they have so many
points in common as to their method of spread, prevention, and general
treatment, that what is said of one will with certain modifications
apply to all.

I said "probably" of widely different origin, because, by one of those
strange paradoxes which so often confront us in real life, though the
infectiousness and the method of spread of all these diseases is as
familiar as the alphabet and as firmly settled, the most careful study
and innumerable researches have failed to identify positively the germ
in any one of them. There are a number of "suspects" against which a
great deal of circumstantial evidence exists: a streptococcus in scarlet
fever, a bacillus in whooping-cough, and a protozoan in measles; but
none of these have been definitely convicted. The principal reason for
our failure is a very common one in bacteriological research, whose
importance is not generally known, and that is, that there is not a
single species of the lower animals that is subject to the diseases or
can be inoculated with them. This unfortunate condition is the greatest
barrier which can now exist to our discovery of the causati