This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
疺 combines the sickness radical 疒 with 乏 (lack, scarcity). The exact historical development is uncertain, but it likely expressed a condition of wasting or deficiency.
The sickness enclosure 疒 surrounds 乏 (lack). Picture a body wasting away from illness, becoming thin and lacking strength.
For ホウ, imagine a hollow, thin body: hollow -> ホウ, and the sickness makes the person waste away.