Merek's is a hard thing.
There's so many other things it could be for the OP, but IMO Merek's is likely. I hope OP answers Eggcessive's questions though, it could be something else that's fixable/treatable that has been missed.
I suspect Merek's, but I'm sure there's other possibilities. Avian influenza, lymphoid leucosis, etc. When it happens again, send a bird for autopsy. Just no other way to say for sure what's going on. Sorry you're dealing with this. :(
What are the symptoms they get when they are sick? Do they all get the same symptoms? how long are they sick before they die? Diseases like Merek's can stay in the soil almost indefinitely, I suspect you have one of those. No idea which without further info. If you raise more chickens, and...