Sick chicken

[Dw4407]

I have been losing chickens one day they are fine next day they are lethargic and don't move around a lot act like leg's are not working thanks

 

[Eggcessive]

How old were the chickens you have lost? Were they vaccinated for Mareks? Were they eating, drinking and having normal droppings? The more details you can give the better. If you lose one more chicken, keep the body cold, not frozen, and contact your state vet to do a necropsy. That is the best way to get a diagnosis. Here is a list of most state vets:
https://www.metzerfarms.com/poultry-labs.html