Here it is free: http://www.gutenberg.org/etext/5200

I was trying to find another story that reminded me of which I thought was by William Carlos Williams or C.P. Snow (still think it might be by Snow but I can't find it) where a boy is sick in bed and keeps telling the doctors something is taking over his body but everyone keeps poo-pooing him and saying it's just his illness and fever and it keeps getting "higher" (started at his feet) and he's terrified and desperate but eventually he wakes from sleep and is wholly well but he's "something" else, the thing totally took him over and no one knows.