Eggcessive provided some informative links. Coryza is a secondary symptom of Chronic respiratory disease. Sulmet (sodium sulfamethazine) is the common treatment for it. Once the foul odor clears up and there is still runny eyes, nose, throat rattles, then you must change medication. Tylan is commonly used for CRD. if Coryza becomes a problem in the flock, it is recommended to vaccinate. The vaccine is drawn from the bottle and kept in the refrigerator. It is .5 cc anywhere under the skin. It is done again 3-4 weeks later. Much should be considered regarding viruses; i.e. the origin of birds, what viruses are common in your area. A state lab would be a good source of information: http://www.aphis.usda.gov/animal_health/animal_dis_spec/poultry/downloads/labs_app.pdf