t of drainage, may also act, by
polluting the ground, and so providing a favourable soil for the germ,
though it is to be noted that "the steady increase in the diphtheria
mortality has coincided, in point of time, with steady improvement in
regard of such sanitary circumstances as water supply, sewerage, and
drainage" (Thorne Thorne). Cats and cows are susceptible to the
diphtheritic bacillus, and fowls, turkeys and other birds have been
known to suffer from a disease like diphtheria, but other domestic
animals appear to be more or less resistant or immune. In human beings
the mere presence of the germ is not sufficient to cause disease; there
must also be susceptibility, but it is not known in what that consists.
Individuals exhibit all degrees of resistance up to complete immunity.
Children are far more susceptible than adults, but even children may
have the Klebs-Loffler bacillus in their throats without showing any
symptoms of illness. Altogether there are many obscure points about this
micro-organism, which is apt to assume a puzzling variety of forms.
Nevertheless its identification has greatly facilitated the diagnosis of
the disease, which was previously a very difficult matter, often
determined in an arbitrary fashion on no particular principles.

Diphtheria, as at present understood, may be defined as sore throat in
which the bacillus is found; if it cannot be found, the illness is
regarded as something else, unless the clinical symptoms are quite
unmistakable. One result of this is a large transference of registered
mortality from other throat affections, and particularly from croup, to
diphtheria. Croup, which never had a well-defined application, and is
not recognized by the College of Physicians as a synonym for diphtheria,
appears to be dying out from the medical vocabulary in Great Britain. In
France the distinction has never been recognized.

Prevalence.

Diphtheria is endemic in all European and American countries, and is
apparently increasing, but the incidence varies greatly. It is far more
prevalent on the continent than in England, and still more so in the
United States and Canada. The following table, compiled from figures
collected by Dr Newsholme, shows how London compares with some foreign
cities. The figures give the mean death-rate from diphtheria and croup
for the term of years during which records have been kept. The period
varies in different cases, and therefore the comparison is o