A respiratory disease may be caused by a virus, in which case you cannot treat it. With viral diseases, you can only wait for the disease to pass, and give supportive treatment. Give electrolytes and probiotics, and make sure that they eat and drink enough.
You could try giving antibiotics, as some respiratory diseases are caused by bacteria, which can be treated. Get some Tylan50, as it is one of the best antibiotics for a respiratory disease. The dosage is 1cc for large fowl, .5ccs for bantams, given for five days. You inject it into the breast muscle using a small syringe and needle (20-22gauge needle is good) once daily, switching breast sides to prevent too much pain/muscle damage.
Other antibiotics like Oxytetracycline can be used, but do not usually work as well as Tylan.
Tylan200 can be used, but dosing it is difficult because it is four times stronger than the Tylan50. There is some controversy over the correct dosage; however, here is what I believe to be the correct dosage:
About .125ccs for bantams (1/5 the amount of Tylan50), and .25ccs for large fowl (once again, 1/5 the amount as it wold be if you were using tylan50).