The agent that causes Hansen’s disease is an acid fast rod-shaped bacillus Mycobacterium leprae. The organism multiplies very slowly (dividing approximately once every 13 days) and is an obligate intracellular pathogen that lacks several genes needed for independent survival, thus it has never been grown in bacteriologic media. However, it has been grown in mouse foot pads by injecting ground tissue from lepromatous nodules or nasal scrapings from leprosy patients into the foot pad of the animal. Typically, the granuloma appears at the inoculation site within 6 months. Armadillos can also be experimentally infected and will develop systemic disease, and are now the most common animal used to study Hansen’s disease and its treatment.