prolonged fever, chronic malaria
瘈 centers on a lingering, wasting illness: a prolonged fever or chronic malaria that slowly consumes the body.
瘈 combines 疒 (sickness) with 契, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a disease.
The sickness enclosure 疒 wraps around 契, like a chronic illness that binds a person in a long-term contract with fever.
For ケイ, imagine a person with a prolonged fever trying to say 'okay' but only managing a weak 'kay': kay -> ケイ, as the fever drags on.