There are alot of illnesses that affect chickens. I’d suggest you google "poultry diseases" & see the symptoms that match the sick chickens behavior. West Nile is a continuing concern- it can wipe a flock out completely. I had it come into my area after the hurricanes on the Texas coast…the chicken acts lethargic & groggy…thay quit eating & sit around with their heads drooping & their feathers ruffled…getting weaker & weaker- until they finally fall over & die. It doesn’t have a treatment as far as I can find, but if a person can protect their poultry from mosquito bites- then that will prevent them being infected….but mosquito to bird- to mosquito is the main source of infection. My horses are vaccinated against West Nile, but the Vet’s say it’s not cost feasible to come up with a vaccination or treatment for chickens. Go figure…