A group of lepers in Jerusalem, 1900. Leprosy, also known as Hansen's disease (HD), is a chronic infection caused by the bacterium Mycobacterium leprae and Mycobacterium lepromatosis; skin lesions are the primary external sign. Left untreated, leprosy can be progressive, causing permanent damage to the skin, nerves, limbs and eyes. This group was photographed and treated by the American Colony in Jerusalem, established in 1881 by members of a Christian utopian society led by Anna and Horatio Spafford. The society engaged in philanthropic work amongst the people of Jerusalem regardless of religious affiliation, and gained the trust of the local Muslim, Jewish, and Christian communities.