Common in 瑕疵 (flaw, defect) and the idiom 吹毛求疵 (to blow apart the fur to find a flaw — to nitpick). It means a small, minor imperfection, not a major fault.
The sickness radical 疒 wraps the top-left. A flaw is treated as a 'blemish/ailment' of an object — extending the illness radical metaphorically, like 病 (illness).
