I have had a few effected by something resulting in death after 3-4 days. Chicken becomes lethargic and loses weight fast. Sometimes can be seen gasping. I have given vitamins and have wormed the flock. I'm fairly confident it's not mareks. I have not used antibiotics....I've been trying to avoid that but I am at the stage now where I will try anything. I haven't lost too many but with loss to Hawks, minks and feral cats I don't want a disease issue too!