The organisms are similar and related (mycobacteria), and there is a curious epidemiologic relationship as well - as tuberculosis became more prevalent, leprosy became more rare.
Both leprosy and tuberculosis were prevalent in Europe during the first millennium but thereafter leprosy declined. It is not known why this occurred, but one suggestion is that cross-immunity protected tuberculosis patients from leprosy... It could do this because TB appears to confer immunity to leprosy. That is, if one becomes infected with the TB bacterium and one develops a successful immune response to it, that immunity to TB also confers immunity to leprosy as well.
Now
another suggestion has been offered:
"...instead of cross-immunity, those who contracted leprosy in the first place became more susceptible to TB infections, either by acquiring new infections or reactivating dormant infections. Since TB kills more quickly, those with leprosy would succumb more quickly than they otherwise would have, and therefore be unable to spread the disease around."
Found in the
Proceedings of the Royal Society, via
Archaeoblog.
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