Yes and no. Sometimes only one gets it badly enough to show any real effects while the others have better immune systems, so they don't look sick. But that isn't a safe bet!! I'd get the whole flock on liquid Amprol (you'll need to go to a vet, but the Amprol itself should be only a few dollars) if I were you, and get that poor hen a fecal sample to make sure of what is afflicting her. It sounds very much like cocci. She will only die if it gets really really bad, and right now it sounds like she is heading that way, so you should act fast if possible.
-MTchick
-MTchick