Chronic Respritory Disease (CRD) is most likely the culprit.
I have had many chickens with CRD and my advice is to quarantine (isolate) them.. Clean up any area where that bird might have been (water dishes, feces, beddings, etc).
You can try to treat the isolated bird. I use Tylosil 200 injections in the breast muscle if you have that available. I also have used the powdered antibiotics (tetracycline) in the water. Injections are better I think for treating the sickness in a sick bird... I have used the powder to treat the water of the healthy birds just in case. I also have an antibiotic eye treatment because CRD will sometimes cause eye(s) to get infected as well.
I have several chickens survive CRD, but most die if it's severe. I have chickens in my yard that have survived and later bred good offspring. I have been 3 years with no CRD outbreak.
PS... Want to add that I also spray for parasites as they increase the chances of diseases spreading... (I use "poultry protector" a natural orange peel spray). I add apple cider vinegar (or VetRx) to water as well if I am not using antibiotics... I try not to use antibiotics if I can