I think it means the girl is depressed or has some other mental illness, or maybe is dying of some slow disease, and she's getting really bad. The cold fever is her illness, and she made him promise when she was well that if she got really bad, he wouldn't stay to watch her drift slowly away. She's the one that told him to leave when she got cold, and she knew it would take a lot of courage "Put your spine in your back"
I think it means the girl is depressed or has some other mental illness, or maybe is dying of some slow disease, and she's getting really bad. The cold fever is her illness, and she made him promise when she was well that if she got really bad, he wouldn't stay to watch her drift slowly away. She's the one that told him to leave when she got cold, and she knew it would take a lot of courage "Put your spine in your back"