Although leprosy was originally introduced into the Northern Territory in the 1880s, it was not until a decade later that it appeared amongst Aborigines from the Alligator rivers region. It subsequently spread slowly at the periphery involving tribe after Aboriginal tribe until, by 1970, more than 80 different ·groups were affected.
Initially there was a high proportion of infectious cases (those with a poor cell-mediated immunity); later the
relative proportion dropped. At the same time the incidence of new, active cases and the prevalence of active leprosy both declined in the Northern half of the Northern Territory. In the Southern Division the situation was different since leprosy only arrived there in comparatively recent times: there have been relatively few new diagnoses, but the proportion of lepromatous cases has been high, and the incidence, measured over two decades (1951-1970) has risen very slowly. The overall picture of leprosy amongst Aborigines in the Northern Territory now is therefore one in which the incidence and the prevalence of active leprosy have both declined in the north while it continues to spread very slowly at the periphery.