chronic malaria
痁 centers on a persistent, recurring illness: chronic malaria, a disease known for its long-lasting, relapsing fevers.
痁 combines the sickness radical 疒 with 占, which likely provides the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a disease name.
Under the sickness enclosure 疒, the component 占 looks like a fortune-teller's mouth 口 over a divination crack 卜. Imagine a chronic illness that keeps returning, as if a fortune-teller keeps predicting more fevers.
For セン, picture a centipede (a creature with many legs, like the recurring fevers of malaria) crawling under the sickness enclosure: cent -> セン.