Mareks never shows symptoms before 5-6 weeks, and usually it is much later. Cannyountake the chick aside and try to feed some mushy wet feed and egg? Electrolytes or Poultry Cell might also be helpful. If she won’t eat, I would try treating with Corid in case of possible coccidiosis. Coccidiosis causes runny droppings with blood or yellow mucus, lethargy/sleepiness, weakness, and standing around puffed up or hunched. Does the chick have trouble walking, and may e has grown weak due to not eating and drinking? Corid is a ailable at most feed stores, and dosage is 2 tsp of liquid Corid or 1 1/2 tsp of the powder per gallon of water for 5-7 days. For smaller volumes use 1/2 tsp of the liquid per quart waterer.