It smells nasty.
I feared one of mine once had Croyza and it did have a fetid breath. I spoke with a vet at the Gonzales TX diagnostic lab from Texas A&M, and I was beside myself thinking poor innocent bird would have to meet it's death prematurely. The vet said from what I described (despite the odor) that he thought it wasn't Croyza. He advised me NOT to cull. So I didn't. My chicken recovered from what ever was ailing her, and she lived quite a long and productive life IMO. A good layer.
I would suggest at least a phone call to a chicken vet, or a person who knows a lot about chickens. There is a person who will do call backs for $25, and the fee is waived if you purchase any meds from http://www.firststatevetsupply.com/ It is "the Chicken Doctor" Dr. Peter Brown. There are also Vets on line if you want to take that approach. If you don't have a local vet who sees chickens --- or if their bird fee runs into the 100's.
ETA do isolate the one that you think may be ill and follow biosecurity -- just in case.