contagious disease, leprosy
癘 centers on severe, spreading illness: a contagious disease, especially leprosy as a historically feared example.
癘 combines 疒 (sickness) with 萬, which likely contributes a phonetic element. The exact historical development is uncertain, but the structure suggests a disease character built around a sound component.
The sickness enclosure 疒 wraps around 萬, a shape associated with a great number. Picture a disease so contagious it spreads to ten thousand people.
For レイ, imagine a ray of light disinfecting the contagious disease: ray -> レイ, and the sickness is cleansed.