As usual, I am going to be the bucket of cold water. A "respiratory infection" is a generic term, but it includes infectious Coryza, Chronic Respiratory Disease (CRD)/mycoplasmosis, infectious Bronchitis, Infectious Laryngotracheitis, etc, etc. Most of those make a bird a carrier of the disease, even if you treat and alleviate the symptoms--I feel it's imperative that you know this so you will know why your other birds come down sick over and over again if you treat her and put her back in the flock.
Of course, you should not return her to the flock in my opinion and others will disagree with me. If you do, especially not knowing which disease she has, you should not sell your birds to anyone or bring new ones in, called "closed flock".
I'm sorry you're having to deal with this. Have you brought in new birds recently or is she a new bird to your flock?