intestinal worm
蟯 refers specifically to an intestinal worm, a parasitic creature that lives inside the gut.
蟯 combines 虫 (insect/worm) with 堯, which likely provides the sound. The character was created to name a specific parasitic worm.
The insect radical 虫 on the left hints at a small creature, and the right side 堯 looks like a twisting shape. Picture a worm wriggling inside the intestines.
For ギョウ, imagine a wriggling worm that makes you say 'gyow!' in disgust: gyow -> ギョウ, tying the sound to the intestinal worm.
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pinworm infection; enterobiasis