Mishnà su Levitico 13:23

A boil and a burn do not combine, nor do they spread from one to the other, nor do they spread from there to the skin of the flesh, nor does [a nega] on the skin of the flesh spread to them. If they were festering they are clean. If they formed a scale as thick as garlic peel, such is the scar of the boil that is spoken of in the Torah. If they were subsequently healed, even though there was a mark in their place, they are regarded as the skin of the flesh.