It could be one of many different respiratory illnesses. Dry bedding, fresh air, clean water and fresh food help-especially good ventilation, which is very important. But sometimes these things happen regardless of the care provided. So you treat them as best you can, hope for recovery.
We treated her just a couple minutes ago. Her eye is red and glazed with nasty film. Her head also smells. She was seeming very bad, so we hope the medicine takes action in time. Pray for her.
It is starting to sound more like coryza because of the odor. You will need to flush her eye out with saline and clean it with QTips. Then I would get some Terramycin eye ointment, Vetropolycin ointment, or some Vetericyn eye gel from a feed store, and apply it twice a day after cleaning any gunk out. Or you could sacrifice the sick hen, and refrigerate her body to ship or take it to your state poultry vet for a necropsy. That is a good way to find out what disease that is affecting the flock. These chronic contagious diseases are spread by a carrier, either a chicken or by the germs getting brought into the flock. Coryza affects only chickens, but it may be carried in by other birds. I would treat for MG if that is what it is, but for coryza, I would cull any affected hens. Either way, the flock should be closed to new birds, and keep these birds until they are all gone. Most respiratory diseases will only remain on equipment and around the coop for several days once all birds are gone.
Did you read the link about the different diseases posted above? See post 9. All chickens rarely die of coryza or MG. Google some articles on coryza and MG in chickens for more info. They can remain sick for weeks or even months, and diseases can recur if there is stress later on. Coryza can be a terrible disease to deal with, but try to get testing to find out for sure what it is. Try to contact your NPIP or local extension agent for help in testing.