I have not seen coryza before but others say that it causes a rotten odor from the beak from secretions. It is hard to know if culling the one hen would prevent your others from getting it or becoming carriers. You will get all kinds of opinions here about that. Your hen was exposed to the disease (if she has MG or a respiratory disease,) and the others may have been exposed at the same time. It can be brought in by a new flock member, on your shoes or clothes from a feed store or a farm, or by wild birds. She may or may not have another outbreak, but sometimes symptoms can recur during periods of stress, such as a molt or severe cold weather. You could treat her, and she might be okay. Laying can be reduced in hens who have been sick. One thing that could be done if you do cull your chicken, is that you could get a necropsy and testing by your state vet to identify a disease. Think about it, and read all you can, then make a decision.
