Recorded diseases, leprosy was also the first human pathogenic condition of bacterial origin for which the causative agent was identified. Despite these historical records, our knowledge of the biology of M. leprae has lagged behind, in good part because it cannot be grown in culture. The full sequencing of the M. leprae genome, completed in 2001, has created possibilities for the development of new diagnostic tests and treatments for leprosy. Analysis of the M. leprae genome has revealed that it contains fewer than half the functional genes of its closest relative, the tubercule bacillus M. tuberculosis. This “minimal gene set”, the result of extensive gene deletion and decay that have eliminated many key metabolic pathways, renders the leprosy mycobacterium extremely slow in replicating and forces it to an intracellular existence.
A massive international effort was launched to eradicate leprosy worldwide.
In some areas such as the Middle East and Europe, leprosy declined after the late medieval period. One theory for the decline is that it was related to the increasing prevalence of tuberculosis—cross-immunity may have protected patients with tuberculosis from developing leprosy, or the compromised immunological status of patients with leprosy may have rendered them more susceptible to underlying latent tuberculosis infection, which resulted in increased mortality.