Get some tetracyclene (sorry for the misspell) from your feed store. Last fall my chickens started coughing. I read all the posts and became really scared that I would have to cull my chickens. However, my local hatchery, (estes hatchery) has a disease site, and it explains that chickens can get respiratory problems that are NOT always deadly. I bought a bag of tetracyclene, mixed 1/2 a teaspoon per 5 gallon waterer, changed and remixed every 24 hours for 7 days. Within 1 day, no more coughing. That was last fall and everyone is fine. I was scared to death!!! Yes, there are bad diseases out there, but sometimes they just get a cold...........which none of the information sites mentioned.
It is very scarey when chickens get sick, but don't panic......get some antibiotic. I know we all want to raise organic chickens. However, a little antibiotic, just when they are sick, is better than dead chickens. If you want to be organic, throw away the eggs while they are on antibiotic and a few days afterwards.