Could be:
Infectious Anemia
Incidence: common chicken ailment in major chicken producing countries, especially in commercial broiler flocks raised on reused litter.
System/Organ affected: blood
Symptoms: in growing birds sudden deaths of apparently healthy birds, or huddling, ruffled feathers, drawn-in head, pale comb, wattles, and skin.
Cause: unknown, possibly an adenovirus. May be related vitamin K deficiency.
Transmission: ingesting bacteria from droppings of infected birds, spreads from breeders to chicks through hatching eggs, or contaminated litter or equipment.
Prevention: Good sanitation. Healthy birds in clean surroundings will develop a natural immunity.
Treatment: none other than supportive therapy, which can include supplements vitamins B and K.
Coccidiosis (Intestinal):
Incidence: common poultry ailment worldwide, especially in warm, humid weather.
System/organ affected: intestinal tract
Symptoms: in growing birds: droopiness, huddling with ruffled feathers, loss of interest in eating, slow growth and possible weight loss, diarrhea. In mature birds: thin breasts, weak legs, drop in egg-laying, sometimes diarrhea.
Cause: several different species of protozoan parasites.
Transmission: droppings of infected birds; spread on used equipment, feed sacks, shoes.
Prevention: good sanitation, keep clean, dry litter.
Treatment: 1 teaspoon amprolium per gallon of drinking water for five days. Antibiotic treatment guards against secondary infection. Follow treatment with vitamin supplements of A and K.
Link:
http://www.raising-chickens.org/poultry-ailments.html