1-3 year old chickens with poopy bottoms & sickness. What should I do?

We have a flock of 12 hens and 1 rooster, all are buff orpingtons except for 2 Dominiques, plus we have 12 new two month old hens(various types) and a two month old turkey. My problem is showing mainly in the buff hens. They range in age from 1 to 3 years old. This past spring we lost, first a buff hen (3yrs. old) and then our last Rhode Island Red (3 yrs. old), about a month apart. After the red died we started watching the other chickens closely and noticed that our one year old hens(we have 2) had poopy bottoms. They don't show any other signs of sickness though. But a week ago another older buff hen (3yrs) began to get lethargic. I hadn't really noticed any poopy bottom on her. We seperated her to a small cage away from the others. She did not eat or drink much, if anything, for the first few days and was very lethargic. I noticed her comb was very pale too. 2 days ago she began to show more strength, she stands up sometimes, and is eating and drinking some again, though not much, and there is more color back in her comb,but her bottom has become very poopy. I have been watching the droppings under the cage and they are very runny, I do not notice any blood in them though. Our chickens are in a 10X10ft. barn, bedded with staw, and they have a 50x30ft. or so run. We live right in town so the opportunity to let them free range is nill, maybe an hour in the evening before sunset when the weather is nice. Thier fence, which we moved a year ago, is completely bare of grass and we did have a small puddle in it during all the rains in late spring. From the reading I've done I wondered if it was coccidiosis, but my older hens, who should have the immunities built up, are the ones dieing. Is it possible that the one year olds are carriers of the sickness that is killing my older birds, and why are they not showing any other symptoms? They seem to be getting around, eating and drinking fine, and they have had the poopy bottoms for at least a month.(The one year olds were hatched from one of our older buff hens last summer.) I am concerned about letting the new two month olds out with the rest of the flock because of the sickness I'm seeing. They are currently in a small area adjacent to the main barn area, but are really needing more space. We had hoped to butcher the 2 and 3 year olds this fall when the chicks started laying, but now I am concerned about how safe it would be to eat thier meat not knowing what kind of sickness they have. Any ideas on what I may be dealing with and the best way to handle it? Up till this year our birds have been so healthy.

Debbie