Pneumoencephalitis May be the disease. It's also called Newcastles Disease. It's a respitory disease with most 80% of it being fatal. This disease is an airborne disease and CAN INFECT humans and other animals, including the rest of your flock, which most likely has it by now. Wash yourself, your clothes, and everything around the chicken. I'm sorry for your loss. There isn't a cure for this disease in chickens yet. If you have any nasal discharge, mucus in your throat, diarrhea, or feel strange, visit a doctor or a hospital IMMEDIATELY. I had a cousin almost die from this disease. Hopefully you sanitize anything she was near.