Sorry in advance for the long read. I have treated a lot of this disease, and I clean a lot of chickens eyes. I think you would have to neglect the chicken badly and let it crust for days to lose an eye. I have seen cataracts develop when the wound is badly placed behind the eye. I have one survivor that developed a mild cataract that is normal today with sight in both eyes, even if not 100%. Like the other posters quoted, you need some luck because it is a viral infection that doesn't necessarily have a treatment, it just needs to heal.
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Because there is an active inflammation going on, please do not pop the pus out like in that video. Clean ONLY the pus under the eyelid while there is an active inflammation. I let someone try that procedure on a chicken in the video and he nearly killed it. He squeezed out a pus and blood mixture and the pain was so bad I insisted he stop. There should not be any blood, if there is it means the inflammation is still active and you cannot squeeze out inflamed tissue.
Here is the procedure I use to clean the eye. You should clean it in the morning after the pus has gelified over night, and clean it evening before bed. Get a clean container full of water, an eyedropper, and some clean cotton cloth. Secure your chicken on the table, you may wrap her in a towel to prevent the wings from flapping, or just hold her in your arms. Drop a few drops of water on the eye to wet it. Let her shake her head and get used to it. Get it nice and moist. Pus may even fall out, which is what you are trying to achieve. If the eyelid opens and the eye is clean, consider stopping.
Once you get the eyelid dripping wet, try to open it. She may even be able to open it herself. You'll see what I mean because it will be easy to wipe the pus out. Dab the eye gently alternating between a dripping wet towel to keep the eye wet, and a damp or dry towel to pull the pus out. Here I did a picture with my dogs eye to demonstrate how I hold the towel because I am not near my chickens. Use the towel to dig to remove the pus.
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Do not squeeze the pus out of the wound until the infection is gone, or you will be torturing the animal and blood will ooze too. If you see blood, stop because the pain is torturous. Only pus that freely falls out and is around the eye.
If you clean under the eyelid every morning and every night, you will not lose the eye. I have never had to do anything like in that video. Keep that eye clean and let it heal.
She may need help eating if she only has one eye. I follow this procedure with giving her water and force feeding if necessary.