This is so true. I had a three year old hen die of coccidosis last spring the same day she showed any symptoms at all. She was up and around in the morning, hiding in her nest box by noon and dead before 5:00 p.m. She hid her illness until she was so sick she couldn't do it anymore. I had the vet see her and started treatment but it was too late to save her. He took samples and confimred the diagnosis.Chickens do not show symptoms of an illness until they are extemely ill. As prey animals, they can't afford to be sick or, they become someone's supper.
Coccidiosis can kill in hours since it is 'hidden' sometimes until it is too late. Try and get the Corid and then hand dose her. Using an eyedropper, dose her with as much as you would think she will drink in one sitting. Do this every two hours until she is up drinking on her own.
Death by coccidiosis is very unpleasant for the bird, these days I keep Corid on hand and if I even suspect coccidiosis, I treat right away.