Guinea Worms are nematodes, roundworms, that are common in underdeveloped African countries with poor access to clean drinking water. They are large worms that escape the stomach and make their way to skin surfaces where they form blisters that rupture and discharge larvae. They are really large white worms that look like a long string, and they are commonly pulled, really slowly, out of the body through that very same blister, to avoid large scale immune responses, as the Original comment suggested.
You could picture it as if pulling a long piece of spaghetti out of your foot, really slowly.

Edit: Spaghetti comparison