[Antagonist] Parasite

[Roomba]

Had a thought: what if the parasite was only harmed by stuff like calomel or atropine? The side effects are significant enough that you're not going to want to chug that stuff down unless you're sure you have a parasite in you, and any attempt to mass-inject the crew on the off chance of a parasite is not going to be looked kindly upon.

Maybe pentetic acid too - that stuff can be chugged down relatively without issue, but it's rare for anyone to bother making it in significant quantities.