Medical Officers of the Army of India.
51.
constantly present congenial conditions for the factors of the poison, localities
which occasionally do so, and localities which never do so. In localities of the.
first nature, the disease is either constantly present or may occur at any time
granted the presence of the actual factors of the poison ; in those of the second,
all that is necessary for the appearance of the disease is the co-operation of
certain generally-diffused influences and the presence of the actual factors ; in
those of the third any local development of the disease is impossible. The
phenomena of general epidemic diffusion of cholera cannot be accounted for solely
as the result of introduction of the factors of the poison, or by the coincidence
of this with the presence of permanent favourable local conditions. Were diffu-
sion of the factors all that is necessary, there would be nothing to account for the
fact that, whilst conditions of human intercourse remain practically the same
from year to year, general epidemic diffusion of the disease only occurs at
fitful intervals. During periods of general epidemic diffusion there must be an
abnormal diffusion of certain conditions affecting areas which ordinarily do not
furnish an entirely congenial environment for the factors of the poison, in such a
fashion that for the time being they come to do so. In any area in which such
temporary modification occurs, the factors of the poison may or may not pre-exist,
Where they do so, the modification will be all that is necessary to secure the
appearance of the disease, where they do not, it must be attended by their intro-
duction.
In the light of the evidence indicating that in many cases intercourse with
an infected locality does exert an appreciable influence in determining the local
development of cholera in previously exempt areas, the phenomena of epidemic
diffusion cannot be fairly ascribed solely to the action of variations in local con-
ditions in affecting the activity and prevalence of universally-diffused factors ;
but it by no means necessarily follows from this that intercourse with previously
infected areas is invariably necessary. There has been an entire absence of
satisfactory evidence to show that the local development of cholera in and
around Paris during 1892 was in any way connected with importation, and it is
quite possible that it was not, but was merely the result of the abnormal activity
and prevalence of certain endemic factors under the influence of temporary
peculiarities in local conditions. The presence of certain factors in coincidence
with certain local conditions is what is necessary, but it does not follow that the
evidence of their presence should always imply their recent introduction ; it
may merely indicate the coincidence of favourable conditions with their presence ;
the phenomenon may merely be the parallel to the regular periodic fluctuations
in the prevalence of cholera which manifest themselves so conspicuously within
the endemic area. Had the disease been prmarily due to the action of agents
manufactured and multiplied within the human body, its appearance in any local-
ity beyond the endemic area would necessarily have implied fresh importation ;
but if it be due to agents whose proper environment is in localities presenting