It's not that I'm into parasites, it's that this game is incredibly good. And the devs themselves point out that the interactions depicted here are somewhere between parasitic and symbiotic.
Anyway, great gameplay concepts supported by good writing and amazing art, with the only downside being the engine.
Parasite Infection starts as a text-based RPG/Adventure, but only to get you familiarized with lore and parasite mechanics, because this game is mainly a management sim: your character is already infected, not much to do but infect more people. The story supports this really well, and the writing for sex scenes is great. Lots of parasitic body modifications, obviously, but I'd say it's done tastefully, as weird as it sounds: the human form is still there, there aren't any hypersized body parts, etc.
And the amount of people you infect, as well as the parasites you use, also matter for the story progression. Kinda.
The game likes to reset your progress in between chapters. You can spend a lot of time preparing for an important event at the end of a chapter, grind and farm various parasite types for in-game weeks, deal well with every obstacle on your way... only for the good guys to prevail and make you run for your life.
This sounds awful on its own, but it is consistent and explained well enough: you're not zerg or alien, you can't actually fight, so stealth and seduction are your main tools. And, on the plus side, the gameplay mechanics either get expanded or get changed for something substantially different. The locations are different for each chapter as well, so these resets allow you to try new things, kinda like going to a new mission in city building sims.
There are still some problems that this game has, most notably the engine and lack of proper descriptions for some mechanics.
The amount of variables clearly pushes Twine to the limit, so the game noticeably slows down at later stages of some chapters. It's not that big of a deal on its own, but there are moments where you want to spam a single button 20+ times, but have to wait 20+ seconds for Twine to process every click. The UI could be better as well. The dev clearly tried to work around the limitations, but it's still wonky and not that intuitive.
With mechanics, it's hard to tell what certain subtypes of parasites do. They're color-coded, but the only time you can see the effect is when you infect someone, and that's easy to forget with everything else going on.
Still, this is an amazing game that keeps you interested throughout the whole experience, and it deserves extra praise for gameplay alone. Unless you're outright disgusted by the fetishes explored here, it's a title worth checking out.