scabby eruption
疥 centers on a skin condition marked by scabby eruptions, specifically referring to a rough, itchy rash or scabies.
疥 combines the sickness radical 疒 with 介, which likely contributes a phonetic element. The character specifically denotes a scabby skin disease.
The sickness enclosure 疒 wraps around 介, like a scabby rash spreading over the skin. Picture the 介 shape as rough, scaly patches trapped under the sickness radical.
For カイ, imagine a scabby rash that makes you want to cry out "kai!" in discomfort. The sharp sound "kai" cues カイ while the rash itches.