This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 疓 is uncertain. It combines the sickness radical 疒 with 乃, but the original semantic and phonetic roles are unclear.
The sickness enclosure 疒 wraps around 乃, but without a clear meaning, picture a mysterious illness that even the best doctors cannot name.
For ザイ, imagine a sick person sighing 'zai' as they lie inside the sickness enclosure: zai -> ザイ.