This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
痟 combines 疒 (sickness) with 肖, which likely contributes the sound. The exact historical development is uncertain.
The sickness enclosure 疒 wraps around 肖, suggesting an illness. Without a clear meaning, picture a vague, shadowy sickness inside the body.
For ショウ, imagine a show of symptoms: the sickness inside 痟 puts on a show -> ショウ.