Leviticus 13:6-8

6The priest will examine him again on the seventh day, and if the sore has faded and has not spread on the skin, the priest shall pronounce him clean; it is a rash. The person must wash his clothes and be clean.

7But if the rash spreads further on his skin after he has shown himself to the priest for his cleansing, he must present himself again to the priest. 8The priest will reexamine him, and if the rash has spread on the skin, the priest must pronounce him unclean; it is a skin disease.