Depending on what disease they have (chickens do not get colds like humans do), even if they seem to get over the symptoms, they may be carriers for the rest of their lives. Just want you to realize that. That said, there is no way to know if it's bacterial or viral. Viruses can't be eradicated with antibiotics. Some folks use a type of Vicks Vaporub for chickens called VetRx, but that does not fix anything, just may help them breathe--the real downside is that it is so aromatic, it could cause issues itself.
The best thing to do is keep the bird warm and give it extra protein like scrambled eggs. And keep the bird separated from the rest and let its own immune system fight the infection.
I have never had any respiratory illnesses here, but I will not treat them. You could have one tested and see if it's CRD (Chronic Respiratory Disease aka Mycoplasmosis, etc). Otherwise, you are treating blindly and maybe unnecessarily.