34 • Keith Manchester 



in the young below the age of five years. Those infants 

 surviving the primary infection were, in consequence, tuber- 

 culin positive and had "lifelong" immunity to tuberculosis. 

 As noted, the age of exposure toM. leprae is, even today, not 

 known, but it is suggested that, epidemiologically. this may 

 be at a somewhat older age than M. tuherculosis exposure. 

 Thus, exposure to M. leprae may have been, in the popula- 

 tion as a whole, to a people already immune to tuberculosis 

 by previous primary infection. The protection afforded there- 

 by may have prevented the establishment of clinical leprosy, 

 except in those individuals whose immune response was 

 compromised by poverty, malnutrition, or intercurrent dis- 

 ease. Any improvement in these features during the advanc- 

 ing Middle Ages would favor the development of an immune 

 population. Furthermore, as has been noted, the simultane- 

 ous presentation of M. tuberculosis and M. leprae to a person 

 sensitized by previous exposure to M. tuberculosis may pre- 

 vent the development of both infections. In communities in 

 which both di.seases were common, such simultaneous ex- 

 posure may, indeed, have occurred. 



Furthermore, in those individuals who developed clinical 

 disease, the immunity conferred by prior exposure to M. 

 tuberculosis may have caused a "right shift" along the 

 Ridley-Jopling spectrum toward tuberculoid leprosy. This 

 clinical manifestation of M. leprae infection is associated 

 with relative noninfectivity. Thus, this diminution in leprous 

 population infectivity may have contributed to the eradica- 

 tion of the disease. 



It is possible therefore that the rapid decline of leprosy 

 from a population in which the more virulent disease of 

 tuberculosis was both widespread and increasing may be 

 paleoepidemiological evidence of a cross immunity between 

 the two diseases. 



The coexistence and increasing prevalence of the two dis- 

 eases took several centuries to develop. The decline of lep- 

 rosy, consequent upon and subsequent to the increasing prev- 

 alence of tuberculosis, likewise took several centuries to 

 occur. Modem epidemiological studies of these two dis- 

 eases, influenced as they are by effective therapy, lack the 

 prolonged time dimension which only paleopathology, pa- 

 leoepidemiology, and medical history can supply. 



Disease is not solely a phenomenon of the modern world, 

 to be studied only in its 20th century context. Disease has a 

 past, it has a present, and will have a future. No one facet 

 should be studied in isolation and ignorance of the others. 



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Zagreb Paleopathology Symp- t9HH 



