The common cold is an acute respiratory illness (ARI) and the most frequent illness people experience, with economic costs estimated at $40 billion per year (Ann Intern Med 2010;153:769-777). Hundreds of different types of viruses cause the common cold, and etiologic agents include rhinovirus, coronavirus, influenza, parainfluenza, respiratory syncytial virus, adenovirus, enterovirus, and metapneumovirus.