I work in microbiology and only recently learned it's actually a fungus.
Disclaimer: I'm sure I've learned about it in the parasitology classes I've had but that was 15 years ago and since then I've been in bacteriology and sequencing.
It got that name because the welts look like a worm in a circle under the skin. It looks like a raised red ring about 2-3cm in diameter that is usually pretty painful and itchy.