ETIOLOGY.--The study of the etiology of leprosy is intimately
connected with that of its history and geographical distribution. From
the earliest times it was regarded in all parts of the world as a
contagious affection, and efforts were made by the sternest laws of
Church and State to control its spread by segregation, by interdiction
of marriage, etc. No disease has ever been regarded with an equal
degree of abhorrence by mankind; none has received greater attention
from physicians of every age. Within the present century it has come
to be regarded, almost without exception, by the profession as
non-contagious. Peculiarities of climate, soil, and modes of life have
been looked upon as predisposing, exciting, or even essential
influences in its causation; but the widespread distribution of the
disease, with the consequent diversity of diet and customs of living,
its prevalence upon the coast and in interior regions, in high
altitudes as well as at the sea-level, in Iceland as in the tropics,
show that these conditions, however they may affect the course of the
affection, have no direct relation to its causation. The theory of
heredity, as the most plausible explanation, has received its
strongest support in the investigations of Boeck and Danielssen in
Norway, where the disease can be traced for several generations in
families. The same conclusions readily present themselves where the
disease is studied in restricted localities, as in Louisiana and New
Brunswick at the present time, where, as we have seen, it manifests
itself closely in families in different generations. But this is a
narrow point of view from which to study the etiology of leprosy. It
often fails to manifest itself in the descendants of lepers in {788}
such communities, and affects persons in whose families it has never
previously existed. Moreover, in countries where it does not prevail
it not infrequently attacks individuals who have at some time visited
regions where it was endemic, and in the latter places may develop in
immigrants from parts of the world where it has never existed.