I have had 2 hens and 1 rooster get weak and die suddenly. Over the last month my daughter and I would notice a chicken laying down and not eating. We would isolate with food and water in a warmish room but after 1 day they would die. The only symptoms are sudden weakness and possible diarrhea, a lot of debris around vent. One hen I gave 2 shots of tylan( .5 cc once a day IM) and she began to improve, eating and moving, but it didn't last long and she also died. Another hen did improve with the antibiotic treatment but still seems weak. There is no signs of respitory problems and they come from completly different pens with different amounts of birds. They have been vaccinated for Mareks, treated for CRD last fall but I haven't dewormed them yet. Any ideas?