ringworm

Etymology

From ring +‎ worm, being descriptive of what it looks like.

Noun

ringworm (uncountable)

1. A contagious fungal affliction of the skin, characterised by ring-shaped discoloured patches, covered by vesicles or scales.
   • "As listed above, in superficial mycoses infection is localised to the skin, the hair, and the nails. An example is ringworm or tinea, an infection of the skin by a dermatophyte."