This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
癅 combines 疒 (sickness) with 畱, which likely contributes sound. The exact historical development is uncertain, but it may be an older or variant form related to 瘤 (tumor, lump).
The sickness enclosure 疒 wraps around 畱, a shape that resembles a lump or something retained. Picture a sickness that causes a lump to form and stay.
For リュウ, imagine a lump that you (リュウ sounds like 'you') notice under the skin: you -> リュウ, and the sickness enclosure holds the lump inside.