I thought Mareks as a possibility too, but....I would treat for cocci because its something you can do and eliminate it. If you do it fast, you may be able to save your chicks. Corid will not hurt even if its not cocci. I treated mine with the powder Corid I got at Tractor Supply. 1 1/2 tsp per gallon of water. Don't give probiotics while treating with Corid. You can syringe the water to them if they wont drink on their own. Good luck and keep us posted.