chickens are sneezing

[gunny]

I have around thirty NH Reds about one year old. Around two weeks ago I noticed some of them sneezing. More and more of them started doing the same thing and the feed store people told me they had a respiratory infection and gave me a bottle of Sulmet (Sulfamethazine Sodium 12.5% drinking water solution). I have been medicating them for 3 days with no improvement. Any suggestions? I am using pine shavings for bedding, plenty of air circulation, the run is dusty but always has been long before this. I think I saw a little discharge on my rooster's nose when I first discovered the sneezing but they all look clean and dry now. No other behavior changes except my rooster does not crow anymore.