Mishnà su Levitico 13:18

The following bright spots are clean:Those that one had before the Torah was given, Those that a non-Jew had when he converted; Or a child when it was born, Or those that were in a crease and were subsequently uncovered. If they were on the head or the beard, on a boil, a burn or a blister that is festering, and subsequently the head or the beard became bald, and the boil, burn or blister turned into a scar, they are clean. If they were on the head or the beard before they grew hair, and they then grew hair and subsequently became bald, or if they were on the body before the boil, burn or blister before they were festering and then these formed a scar or were healed: Rabbi Eliezer ben Jacob said that they are unclean since at the beginning and at the end they were unclean, But the sages say: they are clean.

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.