I just went through this exact thing. My hen was starved, molting and blind in one eye. She got attacked by a raccoon but not injured ( I heard her screaming and chased it off before it didn't anything but psychological damage). After that she was very scetched out and wouldn't eat if other chickens got too close to her. By the time I isolated her (stupid me) she had a raging cocci infestation. If you start the corid make sure not to give vitamins and don't give too high a dose as diahhrrea can actually prevent the corid from maximum effectiveness (per my vet). My hen didnt make it sadly-hopefully this advice helps you save yours! All the other advice that has been given is great as well! Good luck with your hen!