>>About ten years in, he
>>eats something that agrees with him, and now he's living the
>>immortal's life.
>
>"So you're a symbiote."
>

Well, I wasn't imagining something quite so articulate (Adjusted Wolfe-Dekok of about 65, Spengler flux 0.5), and a bit more stimulus-response based.

"So it's an alien tapeworm?"

"If tapeworms deliberately produced a mix of anti-aging drugs tailored to the person they inhabited, yes. It seems to be very concerned for your health. You get injured, it releases something that enhances your healing; you get sick, it does something that enhances your immune system. Do you know where you picked this up?"

"Haven't a clue."

"Well, I wouldn't recommend getting any limbs cut off or taking a bullet for somebody, but it looks like you're going to live a very long time."

Peter Eng
--
Insert humorous comment here.