to recover from any disease, an epidemic
瘥 centers on the cycle of widespread illness: an epidemic spreading through a population, and the subsequent recovery from that disease. The two given meanings are opposite phases of the same public health event.
瘥 combines 疒 (sickness) with 差, which likely contributes the sound and may relate to the idea of difference or change, fitting the shift from illness to recovery or the spread of an epidemic.
The sickness enclosure 疒 wraps around 差, which can suggest a difference or change. Picture a disease changing a community—first spreading as an epidemic, then changing back to health as people recover.
For サ, imagine a sigh of relief as an epidemic ends: "Sah..."—the sound of recovery. The sigh cues サ while the sickness enclosure fades away.