Mycobacterium leprae is an acid-fast, rod-shaped bacterium (bacillus) which causes Hansen's Disease, also known as leprosy in humans.
It has the dubious distinction of being one of the first organisms known as a human pathogen, and is the second most likely mycobacterium to cause human disease. (The most likely is mycobacterium tuberculosis, the third is mycobacterium ulcerans). The bacterium infects only humans, and, curiously, armadillos, though it grows in mouse foot pads. Recently, the genome of M. leprae was mapped, and it was discovered to have an inordinate amount of dead code in it - that is, DNA which serves no useful purpose.
M. leprae is sensitive to rifampicin, the most broad-spectrum anti-mycobacterial drug, but it does not respond to isoniazid, pyrazinamide or ethambutol, unlike M. tuberculosis and most other mycobacteria.