rapidly
疾 centers on swift, urgent motion: something happening rapidly or moving fast. The given meaning 'rapidly' captures this core idea of speed and quickness.
疾 combines 疒 (sickness) with 矢 (arrow). The arrow likely contributes both sound and the idea of something striking quickly, like a sudden illness or swift motion.
The sickness enclosure 疒 surrounds an arrow 矢. Picture an illness striking as fast as an arrow: 疾 means rapidly.
For シツ, imagine a swift arrow hitting a sheet of paper with a sharp 'shitsu' sound: sheet -> シツ, and the arrow flies rapidly.
disease; illness; disorder; condition; ailment
sprint; dash; scampering
gale; strong wind; swift wind; fresh breeze (Beaufort scale)
illness; disease
allergic disease
viral disease
human disease model (e.g. using experimental animals)
mouse model for human disease
three major diseases (cancer, cerebral stroke, acute myocardial infarction)
Chinese Center for Disease Control and Prevention; CCDC
congenital heart disease
running as fast as one can
systemic disease; systemic disorder
systemic disease; systemic disorder
sickness incurred in line of duty
internal disease
coronary artery disease; coronary heart disease; CAD
coronary-artery disease
respiratory illness
underlying disease; primary illness; underlying condition