Leprosy is caused by a tiny rod-shaped microbe called Mycobacterium leprae. Scientists aren't exactly sure how it is transmitted, but they think it probably spreads when an infected person breathes out or sneezes and releases moist droplets carrying the microbe, or else when a cut in his or her skin is touched by someone [source: NIH].
It may take as long as 20 years for symptoms to appear. But once they do, leprosy attacks the peripheral nerves, skin, upper respiratory tract, eyes and mucus membranes of the nose. A person with leprosy may lose sensation in his or her skin, and experience muscle weakness. Those problems, in turn, can cause the patient to suffer traumatic injuries and eventually lose the use of the hands and feet. Sometimes a patient injures himself and doesn't even know it, as he has no feeling in his hand or foot. As a result, gangrene might set in and amputation of the limb might follow [source: NIH].
Leprosy is spread by prolonged close contact with an infected person, but it isn't that contagious. The bacterium can't live outside a host body for very long. And even if a person is exposed to it, chances are that he or she won't become infected, since about 95 percent of humans are naturally immune to it. And today, it can be successfully treated with common antibiotic drugs [source: HRSA].
But throughout most of history, people only saw a horrible, disfiguring disease that no one knew how to prevent or remedy. Scientists believe that the disease originated in Africa, and may have spread to the Middle East about 3000 B.C.E. [source: Robbins et al.]. It's mentioned in the Old Testament book of Leviticus (chapter 13), which instructs the Hebrews to isolate anyone with a suspicious sore and watch to see whether it spread. If so, a priest would pronounce the person unclean and order that "his dwelling should be outside the camp" [source: New King James Version].
That pattern of shunning leprosy patients continued for many centuries. In medieval Europe, they were forced to wear special clothes and to ring bells to warn others when they passed by [source: NIH]. They had to live in "leper-houses" or hospitals such as one on the outskirts of London that was founded by Queen Matilda in 1118, though they were still allowed to enter the city to beg for alms. Some of these leprosaria were quite well run, with acres of farmland and generous endowments by wealthy landowners [source: Rawcliffe]. But of course, there was no cure.