This is my thought on the matter:
This very well could be pullorum disease but I hope not. This is a very serious disease of domestic poultry especially in young chicks. I am unsure now but until recently anyone with pullorum in their flock who were in the NPIP were required to kill every chicken on the premise that tested positive for pullorum. There is a simple blood test that you can do at home for pullorum. If a chick has it and survives it will become a pullorum carrier for life. See NPIP or the National Poultry Improvement Program in your state.
http://www.dummies.com/how-to/content/common-chicken-illnesses-and-treatments.html
Symptoms: Chicks are inactive, may have white diarrhea with pasted rear ends, breathing difficulty, or die without symptoms. Older birds — coughing, sneezing, poor laying.
How contracted: Viral disease; contracted through carrier birds and contaminated surfaces, clothing, and shoes.
Treatment: Destroy all infected birds — birds that recover are carriers. Most chicks infected will die.
Vaccine available: No vaccine, but there is a blood test to find carriers. While the U.S. is trying to eradicate this disease, buy chickens from Pullorum-negative flocks