contagious disease
瘟 centers on a spreading illness that affects many people: a contagious disease or epidemic.
瘟 combines the sickness radical 疒 with 昷, which likely contributes the sound and may relate to warmth or brewing, suggesting a feverish illness that spreads.
The sickness enclosure 疒 wraps around 昷, like a warm, brewing illness trapped inside the body, spreading contagion.
For オン, imagine an 'on' switch that turns on a contagious disease: on -> オン, and the sickness spreads when the switch is flipped.
fever epidemic