I've had this problem with pet store mice before. Sometimes they just lose weight and die. Unfortunately it is contagious, so it is a good idea to separate her from the others. I hope they haven't already been exposed. If you can already feel her spine she probably won't make it, but there's still hope for the others.
Sometimes antibiotics help, but not always. If you don't have a vet available you can get fish mox over the counter. Dosage for rats can be found here on the Rat Report website. You can reduce the dose based on weight to make it small enough for mice.
Hope she pulls through. If you decide to add anymore mice in the future it would be best to keep them in quarantine (away from the others) for at least a month to make sure they're not sick before you add them into the same cage with the others.
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