Historical declines in tuberculosis: nature, nurture and the biosocial model

The large declines in the incidence of tuberculosis over time in the indust rially developed nations have usually been attributed to natural selection or to socio-economic improvements. Both explanations are beset with problem s, as there is little firm evidence for the occurrence of natural selection of resistance to tuberculosis to any significant extent, and doubts have b een expressed as to whether the incidence of a disease can be directly rela ted to measures of socio-economic change without consideration of the impac t of the many specific public health measures that have been taken. In addi tion, analyses of the changing prevalence of tuberculosis must consider the impact of changing environmental and ecological factors that affect, for e xample, the immunising effect of exposure to Mycobacterium bovis and saprop hytic mycobacteria. It is also necessary to determine whether the causative organism is undergoing evolutionary change, as recent reports suggest.