This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
瘣 combines 疒 (sickness) with 鬼 (demon/ghost), suggesting an illness associated with supernatural or wasting conditions. The exact historical development is uncertain.
The sickness enclosure 疒 wraps around a ghost 鬼, like an illness that makes a person look pale and ghostly. Without a clear meaning, imagine a mysterious wasting disease.
For カイ, imagine a ghostly illness that makes you cry out 'kai!' in pain: kai -> カイ, and the ghost inside the sickness enclosure wails.