You had...a parasite in you.

Yes. Disgusting. Ew ew ew. Get rid of it. All that fanfare but frankly...you couldn't. You had tried all of the usual ones. Deworming. Antibiotics. Antifungals. Every anti-parasite on the market. And it did nothing!

Well, dogs de-wormer (when you got desperate enough to try it) just made the parasite uber horny. Not a fun experience. So many of your clothes got ruined from that. Three days of hiding in your room getting your soul fucked out.

You didn't even know what the damn thing looked like! All you knew is that it was somewhere in your body. Could make tendrils and thick tentacles. Released sticky goo sometimes. And could sorta use chemicals and hormones to manipulate you. And was sentient enough to understand you and react.