Leprosy is a nonfatal chronic infectious disease caused by M. leprae, an obligate intracellular bacterial species indistinguishable microscopically from other mycobacteria. The organism is confined to humans, armadillos (in some locales), and sphagnum moss.

• M. leprae cannot yet be cultured in vitro. The organism has a doubling time in mice of 2 weeks (compared with 20 min for Escherichia coli and 1 day for M. tuberculosis).
• Leprosy, which is associated with poverty and rural residence, is a disease of the developing world; its global prevalence is difficult to assess and is variously estimated at 0.6-8 million.
  • - More than 80% of the world's cases occur in a few countries: India, China, Myanmar, Indonesia, Nepal, Brazil, Nigeria, and Madagascar.
  • - In the United States, ~4000 people have leprosy and 100-200 new cases are reported annually.
• The route of transmission is uncertain but may be via nasal droplets, contact with infected soil, or insect vectors.