ISOLATE THEM IMMEDIATELY !! Get them as far from your other chickens as you possible can. Watch your other chickens for any signs of this and isolate them asap. If you do this when they are roosting at night, it will help you find the ones that are sick. If the ones that are isoated are not drinking, force them with a dropper of water and tetracycline. You can get tetracycline at your feed store. Ask the people what to use on chickens and how much. Do not use the tetracycline for more than a few days and then lay off, but continue with the water. Get some chick starter and feed this. Chickens seem to like this stuff. The poop will be different than if you free range or use chicken scratch, do not worry, it should be a brown color. Finally, when the ones you have start to recover, do not allow them back in the flock for a few days. You can use the tetracycline again, but just for a few days at a time. Give them some time in between to build up their gut bacteria and you can give them yogurt to get this going again. Good Luck and keep looking for what you may have.