Looks like a respiratory disease to me. By now, all of your chickens have probably been exposed, and isolating the sick birds wouldn't accomplish much. Start giving your birds electrolytes, and get some antibiotics. Tylan50 is usually considered to be the best antibioic for a respiratory disease. You should be able to find it a a livestock supply store or feed store. If you can't get Tylan, Terramycin and Duramycin might work too, but aren't quite as strong as Tylan.
Keep in mind that once birds have a respiratory disease, they will always be carriers, and may show symptoms again during times of stress. The antibiotics should get rid of they symptoms, but they will never be cured.