Leprosy was one of the scourges of the Middle Ages–not so much because of scale, but because when a person caught it, their community cast them out.  The lazar house in the Brother Cadfael books, St. Giles, plays a significant role in the series.  In the movie, Kingdom of Heaven, Baldwin IV of Jerusalem is portrayed as a leper, which is historically accurate.  He ruled from 1174 to 1185.  The man who recognized he had the disease (instead of the Baron played by Liam Neeson) was William of Tyre, later Archbishop and Chancellor.  As you can see from the following article, the rest of the movie is entirely fictive as well:  http://en.wikipedia.org/wiki/Baldwin_IV_of_Jerusalem

Leprosy, also known as ‘Hansen’s Disease’, is a contagious disease caused by a bacteria, Mycobacterium leprae, which is why it is curable post-antibiotics.  Left untreated, leprosy is often progressive, causing permanent damage to the skin, nerves, limbs and eyes. Body parts fall off as a result of disease symptoms, rather than the disease itself.  ”

• M. leprae multiplies very slowly and the incubation period of the disease is about five years. Symptoms can take as long as 20 years to appear.
• Leprosy is not highly infectious. It is transmitted via droplets, from the nose and mouth, during close and frequent contacts with untreated cases.
• Untreated, leprosy can cause progressive and permanent damage to the skin, nerves, limbs and eyes. Early diagnosis and treatment with multidrug therapy (MDT) remain the key elements in eliminating the disease as a public health concern.   http://www.who.int/mediacentre/factsheets/fs101/en/

The issue with leprosy in the Middle Ages was partly that is was so disfiguring (although not very contagious), and partly that the Bible ascribes it as a product of divine punishment.  People thus inflicted were driven out of their communities and condemned to wander the countryside, often from one leper house to another.

In today’s world millions of people stills suffer from leprosy.   It is curable, but ignorance and poverty are pervasive and prevent its eradication.  This article:  http://www.nippon-foundation.or.jp/eng/news/20091026GlobalAppealReport.html, is well-meaning, but indicates that the prejudice against people with leprosy and their families continues all  over the world.