Someone mentioned air sac disease. If they mean Aspergillosis, that would be a good guess. They can get that at any age, but with chicks it probably kills them faster. They do not have to have respiratory symptoms.
I don't know what your methods are for keeping chickens, but I would disinfect the coop with something like Virkon. Mine get it from the heat and humidity here, and shavings that may have had to be replaced more frequently.
I would certainly send a fresh dead chicken for a necropsy. Casportpony has a link on her posts to how to do it, and she can give you the address where to send it. You may want to send it to a lab to save your chickens.
I don't know what your methods are for keeping chickens, but I would disinfect the coop with something like Virkon. Mine get it from the heat and humidity here, and shavings that may have had to be replaced more frequently.
I would certainly send a fresh dead chicken for a necropsy. Casportpony has a link on her posts to how to do it, and she can give you the address where to send it. You may want to send it to a lab to save your chickens.