It is possible that the history of Mycobacterium tuberculosis, an organism related to leprosy and one that creates a limited cross-immunity to M. leprae infection, has affected the long-term distribution and incidence of leprosy, as may also have other atypical mycobacterial infections such as scrofula and avian tuberculosis. Increased population density in urban areas facilitates the spread of tuberculosis infection, which may have contributed to a declining incidence of leprosy as cities and large towns appeared. In large regions of West Africa today, leprosy's hold over rural, remote villages increases with distance from a city (Hunter and Thomas 1984), whereas tuberculosis infection, evidenced by positive reactions to a tine or tuberculin purified protein derivative (PPD) test, increases dramatically with population density. There is almost no evidence that leprosy existed in the Western Hemisphere, Australia, or Oceanic Islands before it was introduced from the Old World.

This epidemiological relationship between tuberculosis and leprosy, however, is obscured in individual patients. For despite cross-immunity, the most common associated cause of death in leprosaria is tuberculosis, illustrating how long-sufferers of leprosy lose the ability to combat other chronic infections. Moreover, geographic determinants of leprosy infection alone cannot explain the high prevalence of leprosy in densely settled eastern India. It could be, however, that more successful control of tuberculosis in the region permitted the persistence of leprosy and that stigmatization of lepers effectively delayed treatment of the illness by medical and public health authorities.

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