ague, intermittent fever
瘧 centers on a specific kind of illness: a recurring fever that comes in waves, often called ague or intermittent fever. The core idea is a sickness that strikes in cycles.
瘧 combines 疒 (sickness) with 虐, which likely contributes the sound and may suggest the oppressive, recurring nature of the fever.
The sickness enclosure 疒 wraps around 虐, a shape associated with oppression. Picture a fever that oppresses the body in repeated waves, like a recurring illness.
For ギャク, imagine a sick person making a harsh, invented sound: gyak! The gyak sound cues ギャク while the fever strikes again.