Leviticus 13:18-20

“If anyone has a boil on the skin that has started to heal, but a white swelling or a reddish white spot develops in its place, that person must go to the priest to be examined. If the priest examines it and finds it to be more than skin-deep, and if the hair in the affected area has turned white, the priest must pronounce the person ceremonially unclean. The boil has become a serious skin disease.