The short story I refer to began with a cave in, collapse, or other type of mass accident where only one person lived. The parasite jumped from its dying host into that last living person. -- Im a little unclear of detail beyond this point, but the new host was rescued. Turns out he was already suffering a terminal disease. He did battle with the parasite to the point of ending his life in isolation so it would have nowhere to go. ---- I sure would like to read it again!
A little more detail that might help: The guy was a Dr, I think. Anyway he had good physiological knowledge. He was able use this to surgically thwart the parasite trying to control his body as he took his life. The mental/physical battle was short in duration but VERY well developed in the story. I read it about 12 yrs ago. I thought it was a Stephen King, but havent been able to [re] find it.