measles, sickness
疹 centers on visible skin eruptions, especially the rash of measles. The broader sickness sense is a natural extension of that eruptive illness.
疹 combines 疒 (sickness) with 㐱, which likely contributes the sound and may relate to fine or dense things, suggesting a dense rash.
The sickness enclosure 疒 wraps around 㐱, which looks like a dense pattern of spots. Picture a sick person covered in a fine rash: that's measles.
For シン, imagine a person with measles feeling a sharp shin pain from the rash: shin -> シン, and the sickness enclosure shows the illness.
rash; eruption
measles
hives; nettle rash; urticaria
rubella; German measles
eczema; rash
rash; eruption; ...id
shingles; herpes zoster
prickly heat; heat rash; miliaria
epidemic typhus; louse-borne typhus
pityriasis rosea Gibert
roseola
pimple; papule
infantile eczema; baby eczema
ringworm; serpigo
herpes simplex
acute generalized exanthematous pustulosis; AGEP
syphilid
heat urticaria; thermal urticaria
herpes
rash that accompanies a severe itch; prurigo