Leprosy (or Hansen's disease) is an infectious condition caused by the pathogen Mycobacterium leprae which was first described in 1847. It is characterised initially by a macular rash, progressing to dermal nodules most commonly on the nose, face, extensor sufaces of the limbs and the mucous membranes of the upper respiratory tract. Later there is chronic ulceration, with lymph node and peripheral nerve involvement.

By contrast, the disease process called 'leprosy' in older English translations of the Bible (Hebrew sara'at, Greek lepra) was characterised by white skin and hair, thickening and infiltration of the skin and ulceration (Lev 13:3-28). It could also appear on the walls of homes, on items made from leather and as a mildew on linen or wool. (Lev 14:33-53)

True leprosy does not affect other objects in this way and whiteness of the hair (leucotrichia) does not occur, despite the fact that leucoderma may result from scarring.

The less specific NIV rendering of sara'at as 'infectious skin disease' is probably more helpful given our uncertainty.