Harmless in the sense that they aren’t dangerous, but they are capable of biting. Source: one was in my headphones a few years back and was biting my ear. I initially thought my headphones were just really itchy.
In my headphones!
shudders
I still check my headphones every time before putting them on now
My old biology professor did her PhD on Guinea Worm Disease. She used to tell the nastiest stories about what it was like. Suffice to say I have a healthy fear of worms to this day.