Tzara'at is often translated as leprosy, even though the majority of rabbinic commentators understand tzaraas as being a spiritual affliction.
DovBear mentions an explanation from Robert Alter that seems spot on:
…Robert Alter translates tzaraas as skin blanch, noting that in almost every case tzaraas presents as a loss of pigment. Alter suggests this whiteness of skin and hair reminded people of corpses, likely making them worry that whatever impurity that caused (or was caused by) death was present on the afflicted person. This, Alter continues, is why the remedy for tzaraas is virtually identical to the remedy for corpse contamination.…
Photo: A form of leprosy on a person's thigh.