I need a poison with a particular property, and I don't know how to look it up. The story is set on another planet, so it doesn't have to be a poison found on Earth, but I need particular and plausible effects.
Basically, our poor hero gets hit with a poison-tipped arrow, and it... Does something really bad to him. He's hovering near death for a few days (at least two, but I'm easy), then he appears to be recovering, and well enough so that he's allowed to go home (or at least leave his hospital room for awhile--again, I'm easy). But then he has a relapse, and this time he's definitely on the way out.
Again, I don't need an actual, existing poison, but I need a plausible way that some kind of toxic substance could work like this. I'm willing to go with a virus or possibly a bacteria if I have to, but I need the doc to be able to tell the other concerned characters why s/he missed the fact that the hero hadn't actually gotten farther away from Death's door than the welcome mat, although he seemed to be on the mend.
Any help with this would be great! Thank you.
Edit: You guys ROCK! I have many possibilities now, ranging from poison-mushroom-like toxins to effects similar to a drug overdose, to radioactive arrowheads to vicious parasites! The poor hero is doomed, and I am so happy! Thank you!