?Contagiousness of Leprosy. 263
germs leaving the diseased body. Again, the paths by which
the microbes can enter may be such as only in rare cases
permit of their establishing themselves within the tissues, and
finally, the human body may possess certain protective means
which offer great obstacles to the attacking germ.
A disease may thus scientifically be grouped amongst the
infective and contagious diseases, and yet practically and cli-
nically hardly deserve this name. It is to be regretted that
the modern advances of bacteriology and animal inoculations
have tended to make not only the public, but also scientific
writers, disregard the evidence derived from clinical and epidem-
iological experience. Without, for a moment, under-estima-
ting the importance of bacteriological and animal experiments,
it is necessary to guard against taking a one-sided view of the
matter, a tendency to which, in these times, has often been
only too manifest. Experimenting, as the bacteriologist gene-
rally does, on highly susceptible animals, he easily runs the
risk of arguing beyond his premises, and of drawing conclusions
from his experiments which he applies without sufficient re-
serve to the natural mode of infection. It is quite impossible
to deduce the tiology of an infective disease from bacterio-
logy alone; clinical and epidemiological observations carefully
made must always be taken into account. A susceptible ani-
mal may easily be infected by experiment, and yet the case
may be quite different with man.
The best example to elucidate this point will be found in
tuberculosis. The researches of Koch have shown that this
disease is not only infective but also contagious. It is, how-
ever, equally certain that in the ordinary human surroundings
the conditions necessary for the multiplication and proliferation
of the tubercle bacilli never exist. The diffusion of the disease
depends thus, firstly, on its transmission from one individual to
another, and, secondly, on the fact that the bacilli may remain
dormant, in the full possession of their virulent properties, for
a considerable time, without, however, as was just mentioned,