I have lost 3 hens each about 3 weeks apart. Is there a disease that would move that slowly through my flock? The first on I thought worms but not sure. The second one I think had sour crop. I could feel grain in it and she had an awful smell. I massaged it, gave her oil, tried apple cider vinegar after about a week she died, Now today I see another chicken is is listless and hardly walking around. All of my hens are between 4 and 8 years old. The one today I think is only 4 . I give apple cider vinegar once a week in their drinking water and they get DE in their grain once a month. They are free range and all the others seem healthy. I would appreciate any ideas.