Sounds like a respiratory disease Princessnym. Have your flock been immunised for Infectious Bronchitis or Mycoplasma, both of which will sweep through a flock? Have you bought any new birds in within the last two weeks which have been immunised with live viruses for IB, ILT and Myco? The ex-batt will perhaps have been immunised for IB and ILT but maybe not Mycoplasma and that's why she is responding.
Myco is treated with Tylan in the drinking water or a single injection, but if it isn't caught quickly birds rapidly reach the point of no return.
We accidentally infected our flock with IB and ILT from hybrids bought in carrying the latest strains of live viruses and the vet prescribed Synulox. Two stressful weeks later and they had all recovered with no losses. Don't think it would have been the same if they had caught three viruses simultaneously, even the mild inoculation ones.