Leprosy

The “leprosy” of Leviticus (Hebrew tzaraat) is not the modern disease but a range of surface conditions — on skin, cloth, even the walls of a house — and the most severe of the uncleannesses, spoken of as a kind of living death (Num 12:12). Unlike most defilements it had no wash-and-wait cure; the leper had to be pronounced clean rather than scrub himself clean (Lev 14).

See also: What He Was Willing to Touch.