pimple, sore, boil
癤 centers on a localized skin eruption: a pimple, sore, or boil. The meanings all point to a small, inflamed spot on the skin.
癤 combines the sickness radical 疒 with 節, which likely provides the sound and may suggest a node or joint-like swelling, fitting the idea of a boil or pimple.
Under the sickness canopy 疒, imagine a bamboo node 節 swelling up like a painful boil on the skin.
For セツ, picture a painful boil forming a small set of bumps: set -> セツ, and the sickness enclosure holds them in place.
furuncle; boil