The fraction of exhaled nitric oxide (FeNO) in expired air is a reliable measure of airway inflammation and has been used as a marker in asthma and other respiratory illnesses such as primary ciliary dyskinesia, bronchopulmonary dysplasia (BPD), liver cirrhosis, chronic obstructive pulmonary disease (COPD), cystic fibrosis (CF). Although, some exquisite bench research experiments have demonstrated stimulation of nitric oxide production in respiratory epithelial cells infected with RSV, there is a paucity of clinical data regarding levels of feNO in viral respiratory illness and specifically RSV.