The disease is caused by Micobacterium Leprae discovered in 1873 by Armauer Hansen in Norway. It is acid fast bacilli and on staining by Ziehl – Neelsen method the organism appears rod shaped and deep pink in colour. M. Leprae have revealed five properties which are of epidemiological importance
- Man is the known host.
- Optimal growth of the organism occurs at 30°C. This explains its predilection for the skin and upper respective track.
- Stability: It remains viable outside the body for several days particularly under humid conditions.
- Slow growth: The generation time of the organism is 12 to 14 days. Making it the slowest growing bacterial pathogen. This explains the prolonged incubation period and the protracted course of the illness.
- Antigen city: It shares common antigenic determinants with many other micobacteria.