My favorite parasite story is in Episode 404 of the "This American Life" podcast, act 3. It's about a guy who has terrible allergies (to pollen) and fixes it by intentionally infecting himself with hookworms. It somehow calms down his immune system. For a while he sells hookworms to other people. Beautiful story.
It's obviously better if you listen to it, but you can also find it in the transcript: https://www.thisamericanlife.org/404/transcript - search for "act three"